New Opportunistic Aggression

Foreword

With the ongoing war in Ukraine, growing conflicts in the Middle East, and potential for war with China or North Korea, the United States and its allies are facing an enormously complex geopolitical landscape — one further complicated by the changing ways in which adversaries are leveraging nuclear weapons. As the risk of a large-scale conflict increases, particularly with China or Russia but also with North Korea, so too does the potential for nuclear weapons to be used, meaning each individual conflict carries increased risks of nuclear use. At the same time, increased cooperation among China, Russia, and North Korea makes it more likely U.S. adversaries could coordinate or take advantage of potential conflicts with the United States or its allies, increasing the likelihood U.S. forces will have to fight a two-theater or two-front war. Without sufficient conventional forces to deter conflicts from breaking out in the first place, the United States may be forced to rely more heavily on nuclear weapons to deter or respond to aggression on a second front, whether Washington intends to or not.

Taken together, these dynamics increase the risk that nuclear weapons will be used in a future conflict — an outcome the United States has tried to avoid for more than 75 years. Managing these challenges requires deeper interoperability with U.S. allies, a clear prioritization of U.S. resources, and a willingness to make tough decisions about where to accept risk. It also requires a better understanding of how opportunistic aggression — or the possibility a second adversary could confront the United States and its allies while they are already engaged in a conflict elsewhere — would complicate U.S. and combined force planning in new and unexpected ways.

This is why the Project on Nuclear Issues (PONI) at the Center for Strategic and International Studies (CSIS) convened an inaugural Mid-Career Cadre Task Force on Understanding Opportunistic Aggression in the Twenty-First Century. This effort was designed to showcase the exceptional talent and expertise of members from the Mid-Career Cadre and generate new insights on one of the central challenges associated with deterring two nuclear peers.

This report should serve as a wake-up call for senior leaders. War is not inevitable, but if it occurs, the United States and its allies will face very real constraints on how they can deter and respond to aggression on a second front. Such scenarios could place significant demands on U.S. nuclear forces in a way that many policymakers today do not appear to appreciate. Understanding what decisions senior leaders might be asked to make in certain scenarios and what forces might be available — or not — could have a significant impact on future planning, resourcing, and policy decisions. Toward this end, the task force identified several recommendations that deserve serious consideration and action, from establishing a statutory nuclear policy council to developing an engagement plan for discussing opportunistic aggression scenarios with U.S. allies.

We cannot underestimate the enormity of the challenges the United States is facing or the difficulty of the decisions senior leaders must make now and in the coming years. CSIS and PONI are grateful for the task force’s time and energy on this important issue.

Kelsey Hartigan

Deputy Director, PONI

Senior Fellow, ISP, CSIS

May 2024

Task Force Introduction and Overview

Key Questions and Scope

The United States faces an increasingly contested strategic environment that raises significant, unanswered questions about the role nuclear weapons will play in U.S. national security strategy going forward, and the forces that will be required to deter — and, if necessary, defeat — adversaries. For the first time in its history, the United States must contend with not one but two major nuclear powers. This poses several challenges, chief among them how the United States and its allies might confront simultaneous or sequential high-end conflicts with multiple nuclear-armed adversaries. As policymakers grapple with the “two-near-peer” challenge, a crucial question remains: How should the United States mitigate the risk of opportunistic aggression?

To explore these issues and generate new thinking on a central issue affecting future nuclear strategy and posture, the Project on Nuclear Issues (PONI) at the Center for Strategic and International Studies (CSIS) launched the Mid-Career Cadre Task Force on Understanding Opportunistic Aggression in the Twenty-First Century. For the purposes of this report, opportunistic aggression refers to the use of force against the United States or its allies while they are already engaged in a conflict. The task force differentiates between opportunistic aggression and the broader concept of opportunism. While opportunistic states could take advantage of a distracted United States in many ways (e.g., economic actions), actions that fall short of launching an attack are not opportunistic aggression and deserve attention elsewhere.

The group, comprising mid-career experts from both governmental and nongovernmental organizations, represents a diversity of views and backgrounds. Through a series of four multiday intensive workshops in the first half of 2023, the task force explored how the United States and its allies can best deter opportunistic aggression; what, if any, changes to force structure and posture are needed to address the two-near-peer problem; and how planning for opportunistic aggression scenarios in the future might affect extended deterrence commitments, strategic stability, and arms control efforts. Discussions centered on four overarching questions:

• What challenges does opportunistic aggression create for the United States and its allies?

• If the United States is engaged in a conventional conflict against a nuclear-armed adversary in one theater, how might that affect U.S. planning and force posture in a second theater, and what role might nuclear weapons play in each contingency?

• What are the implications for U.S. allies, and how might this affect extended deterrence commitments in the future?

• Does the United States need to be prepared to fight two simultaneous nuclear wars? How might this affect arms control and strategic stability efforts?

This report is the culmination of the group’s effort to outline key findings and recommendations for policymakers to consider.

The task force focused primarily on the implications for U.S. nuclear strategy and forces. It did not focus on what specific steps the United States and its allies might take to strengthen conventional deterrence and defense in both the European and Indo-Pacific theaters, nor did it seek to engage in the growing debate on whether and how U.S. support to Ukraine affects the military’s ability to wage a potential conflict against China — a subset of the broader challenges posed by opportunistic aggression. Strengthening conventional deterrence and nonnuclear planning and operations, as well as the broader U.S. defense industrial base, is critical, but the specific parameters for doing so are outside the scope of this report.

Summary of Findings and Recommendations

Deterring and potentially responding to two separate conflicts or contingencies presents unique challenges, depending on who initiates or takes advantage of a conflict. The task force determined that understanding the specific resource conservation and distribution challenges the United States and its allies might face, as well as the reduced response capability and insufficient resource challenges certain scenarios might bring about, will be critical to identifying and better preparing for the most consequential opportunistic aggression scenarios.

Understanding the specific resource conservation and distribution challenges the United States and its allies might face, as well as the reduced response capability and insufficient resource challenges certain scenarios might bring about, will be critical to identifying and better preparing for the most consequential opportunistic aggression scenarios.

Understanding when a leader might initiate aggression or use nuclear weapons — and what might deter them from doing so — is one of the most difficult intelligence requirements. Assessing how a leader’s calculations might shift if the United States and its allies are already engaged in a conflict elsewhere complicates this matter significantly. The task force identified four key factors in determining the likelihood a state will opportunistically initiate an aggressive action while the United States is already involved in a conflict. An adversary’s perception of a significant change in U.S. and allied capabilities, will, or alliance strength could lead an adversary to determine that there is an opportunity to initiate aggression to achieve its objectives. Changes or failures in signaling could also lead to this conclusion. Improving collection on and assessment of these dynamics will be essential and will allow the United States and its allies not only to better anticipate when opportunistic aggression might occur but also to provide a critical baseline for better tailoring deterrence strategies and understanding how those strategies might interact.

In analyzing the wide range of possible opportunistic aggression scenarios the United States might face, the task force concluded the following:

1. Policymakers appear unaware of the risks they are accepting in regard to opportunistic aggression and the demands certain scenarios could place on U.S. nuclear forces in the impending two-nuclear-peer threat environment.

2. Opportunistic aggression could create unique challenges for U.S. nuclear forces.

3. Current planning for opportunistic aggression scenarios is insufficient.

4. There is insufficient awareness of how threats in one theater might affect U.S. allies’ security interests in a separate theater and the ability of the United States to meet its extended deterrence obligations.

5. It is not clear whether the necessary processes and structures are in place to make difficult and informed prioritization decisions across the nuclear enterprise and broader defense industrial base.

To address these challenges, the task force recommends the following:

1. Policymakers and senior leaders should review intelligence priorities to ensure adequate intelligence collection and assessment on what factors might influence the likelihood a leader will initiate aggression if the United States is already involved in a conflict.

2. Congress should work with the administration to establish a statutory Nuclear Policy Council to serve as a bridge between operational and acquisition-focused decisions and broader National Security Council–led interagency deliberations.

3. The Department of Defense (DOD) should prioritize war games, tabletop exercises (TTXs), and scenario-based discussions that focus on opportunistic aggression scenarios and facilitate interagency, allied, and outside expert participation, where possible.

4. The Departments of State and Defense should develop an engagement plan for allies to empower more detailed and transparent conversations on opportunistic aggression scenarios.

5. The executive branch and Congress should address critical conventional shortfalls and work with allies to devise complementary investment strategies.

6. Departments and agencies should prioritize nuclear deterrence education for senior leaders.

7. The DOD and National Nuclear Security Administration (NNSA) should diversify U.S. nuclear forces to ensure the president has a full range of options if deterrence fails in one or more theaters.

8. Policymakers should accept calculated risks and ensure the necessary processes and structures are in place to make difficult prioritization decisions. They should also allow the advanced development communities within DOD laboratories and NNSA national laboratories to experiment more aggressively.

Defining the Problem

Force Planning and the Challenges of a Two-Front War

For the first time in its history, the United States must contend with not one but two major power rivals with nuclear weapons. As policymakers grapple with how to manage this two-near-peer or tripolar deterrence challenge, a key question is how the United States should approach the prospect of opportunistic aggression.

As the 2022 Nuclear Posture Review (NPR) points out:

Opportunistic aggression could create deterrence challenges. Should we find ourselves in a large-scale military confrontation with a major power or regional adversary, the Joint Force will need to be postured with military capabilities — including nuclear weapons — that can deter and defeat other actors who may seek to take advantage of this scenario to engage in opportunistic aggression.

To address these challenges, the NPR and National Defense Strategy (NDS) outline a broad force planning construct:

To deter opportunistic aggression elsewhere, while the United States is involved in an all-domain conflict, the Department will employ a range of risk mitigation efforts rooted in integrated deterrence. These include coordination with and contributions of Allies and partners, deterrent effects of U.S. nuclear posture, and leveraging posture and capabilities not solely engaged in the primary warfight — for example, cyber and space. Additionally, the Joint Force will be shaped to ensure the ability to respond to small-scale, short-duration crises without substantially impairing high-end warfighting readiness, and to conduct campaigning activities that improve our position and reinforce deterrence while limiting or disrupting competitor activities that seriously affect U.S. interests.

Unfortunately, there is little evidence the United States is prepared for such scenarios. Due to policy and strategy decisions made years ago in very different security environments, today’s Joint Force is not sized or shaped to address two simultaneous or sequential high-end conflicts borne out of opportunistic aggression scenarios. In addition, the United States already struggles to fully integrate with allies when preparing for a single crisis or conflict, let alone work through the implications for U.S. allies if large elements of U.S. conventional forces are tied up in a separate theater and the United States is forced to rely more heavily on its nuclear forces and allies to confront a second aggressor. Addressing these challenges requires policymakers to first understand what risks they are accepting today and in the future as a result of strategy and force structure decisions made years ago.

The notion that the United States needs to be prepared to prosecute two simultaneous conflicts is not a novel concept. After the Cold War, the United States could fight regional wars in places like Iraq and Afghanistan while maintaining sufficient capabilities to deter military adventurism by other adversaries. In fact, multiple presidential administrations reaffirmed that the United States must be able to prevail in two simultaneous (or near simultaneous) conflicts, including President George H. W. Bush in his Base Force concept and President Barack Obama in his defense strategy. But a decade ago, experts and officials often discussed two-war force sizing in the context of two simultaneous medium-sized or regional conflicts. In 2014, then secretary of defense Chuck Hagel asserted that U.S. forces, provided they were funded at the levels called for by the administration’s pending budget request, would be “capable of simultaneously defending the homeland; conducting sustained, distributed counterterrorist operations; and in multiple regions, deterring aggression and assuring allies through forward presence and engagement.” He went on to state that if deterrence should fail, “U.S. forces could defeat a regional adversary in a large-scale multi-phased campaign and deny the objectives of — or impose unacceptable costs on — another aggressor in another region.” Soon, however, the growing threat from China changed this thinking.

In 2018, the NDS adopted what was functionally a one-war force-sizing construct and recommended only modest increases in force capacity. This approach recognized the reality of what it would take to execute a high-end war against a large nuclear-armed adversary such as China or Russia but arguably created operational and strategic vulnerabilities. As noted by the 2018 NDS Commission,

The Department has largely abandoned the longstanding “two-war” construct for a “one major war” sizing and shaping construct. In the event of a large-scale conflict with Russia or China, the United States may not have sufficient remaining resources to deter other adversaries in one — let alone two — other theaters by denying them the ability to accomplish their objectives without relying on nuclear weapons.

This warning largely went unnoticed, however, in part because it seemed unlikely the United States would be forced to confront simultaneous conflicts. Moreover, the United States intended to rely on its allies’ growing conventional capabilities and increased burden sharing to offset some of the risks a two-theater or two-front war might entail.

But as this report details, the changing geopolitical environment, the presence of two powerful nuclear-armed adversaries, the growing threat from states like North Korea, and the increased cooperation among those states means that it may be infeasible to conventionally deter one adversary while committing sufficient conventional capabilities to fight and win against the other — let alone fight and win against both simultaneously. This has significant implications for U.S. nuclear forces as well as for U.S. extended deterrence guarantees.

Addressing major-power opportunistic aggression in a second theater requires understanding what a large-scale conventional conflict (or conflicts) might involve and how such a conflict would constrain U.S. ability to flow forces and support logistics into a second theater or area of operations if a second adversary initiated aggression on a separate front. It also likely requires some combination of greater optimization of U.S. and allied conventional force structures, increased allied force contributions, improved industrial base efforts to reconstitute critical conventional capability deficiencies as well as the nuclear enterprise, and improved U.S. theater nuclear capabilities and integration. Deconflicting and prioritizing these efforts will be essential.

Opportunistic Aggression in the Current Strategic Environment

The next decade may be among the most strategically challenging in U.S. history. Facing the prospect of two near-peer competitors for the first time in the nuclear era, the United States continues to maintain significant global commitments. These include managing a complex network of alliances, including promises to protect these allies against aggression and potentially use U.S. nuclear weapons to defend them. While the Joint Force remains the most capable conventional military organization on the planet, this complex strategic picture entails myriad contingencies that could require the United States to commit or expend some or nearly all of its nonnuclear capabilities.

If the United States were to commit a significant portion of its conventional forces to a war, it would necessarily reduce the strength of its conventional deterrent against other adversaries, who might take advantage of perceived U.S. weakness or distraction. For instance, a series of 24 war games conducted by CSIS indicated that the United States would likely flow a significant portion of its forces to the Pacific in the event of a conflict with China over Taiwan. And even if the United States, Taiwan, and Japan were successful in repelling a Chinese invasion, the United States would likely still lose “dozens of ships, hundreds of aircraft, and tens of thousands of servicemembers.” The United States would also likely run out of long-range precision-guided munitions in less than one week during a Taiwan contingency. Amid such hostilities, the ability to respond to aggression in another theater — by Russia, for instance — could be sharply curtailed.

If the United States were to commit a significant portion of its conventional forces to a war, it would necessarily reduce the strength of its conventional deterrent against other adversaries, who might take advantage of perceived U.S. weakness or distraction.

Stockpile shortages resulting from U.S. support for Ukraine have amplified concerns about the state of the U.S. industrial base. These forecasted and current shortages raise serious questions about the speed with which the U.S. defense industry can replenish depleted stockpiles, what stocks would be available to respond to a crisis or conflict in a second theater, and the ability of the United States to provide allies with indirect aid while fighting a major war.

While opportunistic aggression is not a new concern — both the 2018 and 2022 NDSs identify deterring opportunistic aggression as a priority for the Joint Force — the question remains: How can the United States deter other adversaries from striking when it is already committed to a conflict, and what implications might this have for U.S. nuclear forces if deterrence fails?

The two-nuclear-peer problem and the increasing partnership between China and Russia (as well as North Korea) compounds the risk of opportunistic aggression. Not only is conventional deterrence increasingly challenged, but changes in how nuclear-armed adversaries are integrating nuclear coercion and potential use into their theories of victory present unique operational dilemmas for the United States and its allies. Russia’s growing reliance on nuclear weapons amid its conventional losses in the war in Ukraine, China’s unprecedented expansion and diversification of its nuclear forces, and the continued risk of North Korean nuclear use all pose unique deterrence challenges. But the United States cannot deal with these threats in isolation.

The recently released report of the Congressional Commission on the Strategic Posture of the United States took note of this problem in its findings: “The new partnership between Russian and Chinese leaders poses qualitatively new threats of potential opportunistic aggression and/or the risk of future cooperative two-theater aggression.” Indeed, cooperation on military matters between China and Russia — as well as North Korea — has expanded significantly in recent years, particularly since Russia’s 2022 invasion of Ukraine. Even if these adversaries do not formalize their alignment through treaties or deliberately coordinate aggression against the United States, their growing ties complicate U.S. defense planning.

In one of the most overt signs of growing cooperation, Russia and the People’s Republic of China (PRC) have significantly expanded their joint military activities. Since 2018, the PRC has regularly sent forces to participate in Russia’s annual district-level exercises, which focus on high-intensity conventional war fighting, often against fictional opponents that resemble the United States and its allies. Russia and the PRC have also expanded cooperation between their air forces: the Russian and People’s Liberation Army (PLA) air forces conducted their first joint bomber patrol in 2019 and have since continued these patrols, often flying through Korean and Japanese air-defense identification zones. The Russian and PRC navies have engaged in joint exercises since 2012, including an exercise in the Baltic Sea in 2017 and, most recently, an exercise in the Sea of Japan in 2023.

Beyond military exercises, Russia and the PRC are reportedly increasing ties between their nuclear industries and programs. In early 2023, Western media reported that Rosatom — Russia’s state-run nuclear energy corporation — was supplying the PRC with thousands of kilograms of highly enriched uranium to fuel fast breeder reactors. U.S. officials believe these reactors will produce weapons-grade plutonium for the PRC’s nuclear weapons program. Although the precise motivations for this cooperation are unclear, the deal is a further sign of growing military cooperation between the two countries.

Russia has also strengthened its ties with the Democratic People’s Republic of Korea (DPRK) since the 2022 invasion of Ukraine. Russian and DPRK officials have made visits to Pyongyang and Moscow, and the Kremlin has reportedly purchased millions of artillery shells and rockets from the DPRK to support its war effort in Ukraine. The DPRK has also provided Russia with ballistic missiles for use in Ukraine. In return, Russia is assisting the DPRK’s efforts to develop and launch military satellites, though the precise nature and impact of this assistance is unclear.

There are many ways these growing ties could complicate and stress U.S. planning. When detailing plausible conflict scenarios associated with the two-near-peer problem, a study group through the Center for Global Security Research (CGSR) at Lawrence Livermore National Laboratory highlighted that an adversary taking advantage of U.S. engagement in one theater (via crisis or war) may instigate opportunistic aggression in a second region or even pose a joint aggression threat. The report notes multiple possible permutations of opportunistic aggression, with dynamics likely to be context specific. Conflicts could result from inadvertent or coincidental actions, or via actions undertaken by one of the two parties involved in the first conflict. In other words, opportunistic aggression will not play out in predictable or identical pathways but, rather, could be sequential, uncoordinated, coordinated, or even the result of an alliance between the two aggressors. Notably, an adversary may act as though it is preparing to initiate a crisis or conflict, even if it does not intend to do so, merely to complicate U.S. force planning and draw U.S. forces away from a surprise attack by the other adversary.

The United States must be prepared for such scenarios. As the recent Congressional Commission on the Strategic Posture of the United States makes clear:

The objectives of U.S. strategy must include effective deterrence and defeat of simultaneous Russian and Chinese aggression in Europe and Asia using conventional forces. If the United States and its Allies and partners do not field sufficient conventional forces to achieve this objective, U.S. strategy would need to be altered to increase reliance on nuclear weapons to deter or counter opportunistic aggression in the other theater.

While the prospect of a two-front war against Russia and China is a worst-case scenario for opportunistic aggression, it is not the only concerning scenario. Other regional actors such as North Korea or Iran could prove difficult to deter if the United States were to find itself fighting either of its peer competitors. Conversely, if the United States found itself in a war against a minor power, even a limited conflict could impose significant resource constraints on the Joint Force that might create windows of opportunity for a peer competitor state. As a recent Atlantic Council study concluded:

US thinking about war in East Asia often neglects the possibility that the United States would have to fight the PRC and North Korea simultaneously rather than separately. Furthermore, conventional wisdom in the United States underestimates the risk that either the PRC or North Korea would resort to a limited nuclear strike in the event of a conflict in the region.

Addressing these issues requires a more detailed understanding of how U.S. and allied resources might be constrained in various opportunistic aggression scenarios.

Resource Challenges and Opportunistic Aggression

The challenges that opportunistic aggression presents are myriad. However, certain configurations of adversaries are more likely to present challenges than others. Whether the first or second adversary is a nuclear peer matters for whether the United States must conserve or reallocate resources and how much it might be forced to rely on nuclear weapons in its strategy. While U.S. conventional capabilities are likely sufficient to deter most adversaries in peacetime, the prospect of opportunistic aggression is a reminder that the U.S. conventional deterrent might be significantly degraded by even a brief high-intensity conflict. This creates four primary challenges: resource conservation, resource distribution, reduced response capability, and insufficient resources.

Resource conservation challenge: First, in the event of a war with a non-peer state, the United States would face a resource conservation challenge. For example, the Joint Force might be called upon to reserve a significant portion of critical resources (e.g., Long Range Anti-Ship Missiles [LRASMs]) to preserve the conventional deterrent against a Chinese amphibious invasion of Taiwan. Committing (and consuming) high-end or low-density conventional capabilities would diminish U.S. capacity to respond to additional threats. While the NDS appears to acknowledge this dynamic, in practice, combatant commanders will almost certainly push for any and all resources to respond to extant crises, which makes a decision to conserve or withhold certain assets both politically and operationally challenging. Failure to make such a decision, however, could leave the United States with fewer nonnuclear capabilities to deter or respond to aggression on a second front.

Resource distribution challenge: If a second non-peer adversary took advantage of a perceived weakness to engage in opportunistic aggression, the challenge would deepen. In addition to the resource conservation challenge, the United States would face a resource distribution challenge. Capabilities would need to be diverted to the new conflict while maintaining an effective deterrent against adventurism by other actors, especially peer competitors. This might require the United States to adopt riskier strategies to end the non-peer conflicts quickly, accept suboptimal conflict outcomes, rely more heavily on allies, or, if subsequent aggressors are nuclear-armed states, rely more on nuclear weapons to deter further opportunistic aggression. Such a scenario could also force the United States to revisit its policy regarding the employment of nuclear threats or operations against Nonproliferation Treaty (NPT)–compliant states to provide more flexibility as threats evolve.

▲ Table 1. Resource Challenges and Opportunistic Aggression

Reduced response capability challenge: By contrast, if the initial adversary were a nuclear peer (i.e., Russia or China), opportunistic aggression would pose a different set of challenges. The United States could not afford to reserve significant capabilities in a high-intensity conventional conflict against a peer and would therefore face a reduced response capability challenge in the event of opportunistic aggression by a non-peer adversary. The United States might be limited in its ability to fulfill commitments to allies or might be forced to adopt a more hands-off strategy against the new aggressor to help hold the line, perhaps following the model of current U.S. support for Ukraine. With its conventional forces committed to fighting one peer competitor, the United States may need to lean heavily on its nuclear arsenal to deter the other from entering the fray or taking independent action against U.S. or allied interests. It may also be forced to rely more heavily on allied capabilities and will.

Insufficient resources challenge: Finally, if both the initial adversary and the opportunistic aggressor are nuclear peers, the United States would confront an insufficient resources challenge. Lacking the conventional capabilities to fight and win against two near-peer adversaries simultaneously, the United States would either need to back down (in either or both conflicts) or face the prospect of fighting a conventionally superior adversary in one or both theaters, depending on resource allocation. This strategy would likely require the greatest reliance on nuclear weapons and result in the greatest likelihood of U.S. first use, as the United States might need to employ tactical nuclear weapons to offset conventional weaknesses. Table 1 outlines the four configurations of adversary types and the associated challenges.

While the peer/non-peer status of the initial and opportunistic aggressors helps inform the nature of the challenge, the task force recognizes that opportunistic aggression may take many forms. Although the authors limit the definition of opportunistic aggression to military action against the United States or its allies, different types of attacks, depending on their nature and target, would require different and proportionate responses.

The scope and scale of a U.S. response to opportunistic aggression would also be informed by the nature of the conflict. In accordance with the threat posed to U.S. interests and in keeping with the law of proportionality, the United States is unlikely to respond to a minor conventional incursion with a nuclear strike. Similarly, it is unlikely to defer a response to a case of nuclear use against an ally, even if the United States is already embroiled in a peer conflict. While this report primarily focuses on conventional crises that have the potential to escalate to nuclear use, the task force notes that it is possible a crisis might start with nuclear use — for example, a North Korean nuclear attack on a U.S. ally intended to swiftly achieve capitulation, or even a Chinese or Russian demonstration strike to show resolve and deter U.S. intervention. Such crises would put even greater demands on U.S. conventional and nuclear capabilities in order to successfully restore deterrence and manage (de)escalation.

Factors That Influence Opportunistic Aggression

The task force identified four key factors in determining the likelihood a state will opportunistically initiate an aggressive action while the United States is involved in a first conflict. These factors are capability, will, alliance strength, and signaling.

Capability. An adversary who believes that U.S. capabilities have been so compromised by an initial conflict that the United States could not meaningfully respond to new aggression will demand more, be less willing to compromise, and be more likely to resort to force if its demands are not met. The timing and scale of the initial conflict will determine the extent of this problem. Adversaries are very likely to scale the aggressiveness of their actions based on the severity and visibility of U.S. losses. However, an adversary may be emboldened even if the Joint Force is seeing success on the battlefield if that success has exhausted high-end limited resources like precision-guided munitions or long-range fires.

Will. Similarly, an adversary who believes the United States lacks the will to risk involvement in a secondary conflict may be emboldened to act. The appearance of war weariness in an administration or the U.S. public would send a message that the United States would rather concede than engage in additional conflict. Adversaries would also take cues from the length of the initial conflict, as wars that stalemate and drag on for years are more likely to lose public support over time. Widespread antiwar activism or a rise in populist ideas of isolationism could also signal a waning interest in continued or expanded fighting.

Alliance strength. Opportunistic aggression may not be targeted at the U.S. homeland. Aggressors may view a distracted United States as an opportunity to attack a U.S. ally with potentially fewer repercussions than during peacetime. This factor is exacerbated if U.S. alliance commitments appear to wane as a result of the initial conflict. This might take the form of public statements by the U.S. administration questioning the strength, necessity, or fairness of an alliance, but even the reallocation of resources could send the message, perhaps unintentionally, that the United States has deemphasized an alliance. For example, if the United States pulled capabilities from South Korea to respond to a Taiwan contingency, North Korea might perceive both a weakening of U.S. capabilities on the peninsula and, if done against the protest of the South Korean government, a weakening of the U.S.–Republic of Korea alliance.

Signaling. In addition to the message sent by changes in capabilities, will, and alliance strength, changes in or failures of U.S. signaling might encourage or discourage opportunistic aggression. How Washington handles strategic communications and signaling geared toward other potential aggressors could be critical in shaping perceptions of whether the United States is vulnerable to an attack on a second front. The ability to effectively communicate consequences and distinguish escalation control signals aimed at one adversary from deterrence signals aimed at a second adversary could also shape an adversary’s perceptions of whether the United States is likely to respond in a second theater or on a second front. Escalation dynamics and signals could therefore have a significant impact on an adversary’s thinking.

When Deterring Opportunistic Aggression Fails

Given the challenges of responding to opportunistic aggression, the United States should aim to deter adversaries from opening a second front whenever possible. Maintaining a high level of conventional and nuclear readiness, demonstrating the will to fight, showing strong commitment to allies, and engaging in unambiguous messaging should all contribute to effective deterrence. Should deterrence fail, however, the United States may face the difficult prospect of trying to manage escalation and keep a conventional conflict conventional in one theater while simultaneously relying on the threat (or even employment) of nuclear weapons in a second theater. If, in such a scenario, the United States were forced to employ nuclear weapons — for example, if the United States has just used tactical nuclear weapons to stop a Chinese invasion of Taiwan or respond to a crisis on the Korean peninsula — it would be extraordinarily difficult to argue that Russia should not use tactical nuclear weapons in the face of conventional defeat. The challenge of concurrently managing intra- and inter-conflict deterrence is therefore both understudied and undertheorized but increasingly relevant in a two-near-peer world.

The challenge of concurrently managing intra- and inter-conflict deterrence is therefore both understudied and undertheorized but increasingly relevant in a two-near-peer world.

Findings and Conclusions

The task force identified five broad findings over the course of its deliberations.

Policy Findings

1. Policymakers appear unaware of the risks that they are accepting in regard to opportunistic aggression and the demands that certain scenarios could place on U.S. nuclear forces in the impending two-nuclear-peer threat environment.

Assuming current spending rates and force constructs remain relatively constant, policymakers will be accepting future risks, whether or not those risks are identified and understood. As a result, in the event of future opportunistic aggression, it is unlikely policymakers will have the number of options they assume will be available to them. Significant changes or decisions related to conventional force generation can take years to implement and, once operational, often reflect a security environment from prior years. As such, if U.S. policymakers and decisionmakers do not take action now, they may be forced to rely on nuclear weapons to compensate for shortfalls in conventional capabilities in the future.

The threat of opportunistic aggression may challenge the current U.S. policy of “reducing reliance on nuclear weapons.” In fact, if changes are not made, the current force posture and structure may well require the United States to rely more on nuclear weapons in a second theater or on a second front in the event of a future opportunistic aggression scenario. This threat demands policymakers to address the disconnect between extant policy and current capabilities.

Capability Findings

2. Opportunistic aggression could create unique challenges for U.S. nuclear forces.

As discussed, if the United States and its allies are engaged in a major conventional conflict with a nuclear-armed adversary, the United States may be forced to rely more on nuclear weapons to deter and potentially respond to aggression in a second theater or on a second front. If an adversary perceives a benefit to threatening nuclear escalation in one or both conflicts, it will present even greater challenges for the current U.S. nuclear force posture. Thus, while current capabilities may provide sufficient deterrence in the near term, the current nuclear force structure is insufficient, given current U.S. nuclear strategy, to respond to conflicts with two near peers or in two theaters simultaneously if deterrence fails.

Further, the current mix of U.S. nuclear capabilities is insufficient in its versatility to proactively deter and retroactively respond to opportunistic aggression in a second theater or second area of operations. The nuclear-armed sea-launched cruise missile (SLCM-N) program will help fill certain gaps in nuclear capabilities that are not easily met through any other current or developing option. However, the development and fielding timelines within the nuclear enterprise today do not support closing that gap within the next 10 years. This leaves a significant challenge in the Indo-Pacific theater, where regional nuclear capabilities could play an outsize role in an opportunistic aggression scenario, especially in terms of deterring Chinese or North Korean coercion or escalation. Responding to opportunistic aggression in the Indo-Pacific region after a conflict has already broken out elsewhere — that is, dealing with a second crisis or conflict — would further exacerbate this gap given the lack of forward-deployed nuclear assets in the region and the demand that would already be placed on certain capabilities given the initial conflict. Introducing greater diversity and versatility in modernized nuclear systems, as well as additional options for the extant U.S. stockpile, would provide more credible response options for dealing with acts of opportunistic aggression.

Planning Findings

3. Current planning for opportunistic aggression scenarios may be insufficient.

Combatant commanders must prepare for various opportunistic aggression contingencies, including scenarios that require them to coordinate forces and plans (including deterrence strategies and operations) in new and different ways across multiple geographic combatant commands, including United States Strategic Command (USSTRATCOM). This requires a comprehensive understanding of the forces available — conventional, nonnuclear, and nuclear, including nuclear support assets — and a better process for testing plans and assumptions if the assumed or preferred level of forces is not available.

But this type of planning is limited given the current planning approach. Despite the focus the 2022 NPR places on integrated deterrence, nuclear planning is often segregated rather than integrated within the DOD. Because nuclear plans are segregated by a combatant command (CCMD), geographic combatant commanders plan and execute conventional operations, while nuclear planning is largely relegated to USSTRATCOM. This separation stems from force design: most combatant commands do not have the necessary skill sets and personnel to conduct nuclear planning. Those skill sets mostly reside in USSTRATCOM, though there are some positive changes in United States Indo-Pacific Command (INDOPACOM) — for instance, as the Air Force attempts to bolster its deterrence posture for strategic competition with China.

More detailed and realistic planning is also hindered by the reticence of the DOD — and the U.S. government as a whole — to talk about nuclear planning and operations. Policymakers may need to “think the unthinkable” when it comes to opportunistic aggression, especially for scenarios involving possible low-yield nuclear use. Geographic combatant commanders (GCCs) need to prepare for the possibility that low-yield nuclear use in one theater or situation would affect force flows and decisionmaking in another theater, especially as it pertains to deterrence operations with nuclear forces. Would more or different nuclear assets be needed in one region to maintain deterrence or reestablish it? How might nuclear use change force flows and operations for the conventional side in either theater? Reluctance to work through these difficult issues ahead of time stifles planning and could encourage adversaries to exploit perceived gaps.

Extended Deterrence and Alliance Management Findings

4. There is insufficient awareness of how threats in one theater might affect the security interests of U.S. allies in a separate theater and the ability of the United States to meet its extended deterrence obligations.

Most U.S. allies are understandably focused on potential crises and conflicts on their territory or in their own backyard. Although the North Atlantic Treaty Organization (NATO) began to consider the impact of China’s rise on the security of the alliance in 2022, Russia’s invasion of Ukraine derailed many of these discussions. The United States must lead difficult — but critical — conversations with allies regarding the threat of opportunistic aggression to better prepare for such contingencies. While existing dialogues and formats could enable such conversations, current forums are insufficient or lack the candor needed. Opportunistic aggression will likely require the United States to rely more heavily on allies to respond simultaneously in two theaters. In advance of such an extreme circumstance and in a precrisis setting, the United States can work with allies and partners now to define requirements while balancing assurance concerns. There will be different implications and asks for each ally.

The United States must lead difficult — but critical — conversations with allies regarding the threat of opportunistic aggression to better prepare for such contingencies.

For instance, if large elements of U.S. forces are tied up in the Indo-Pacific theater and the United States is forced to flow certain assets out of the European theater, NATO allies need to understand the potential implications. Likewise, the United Kingdom and France, in particular, may need to be prepared to respond to queries about whether there are specific, credible actions they could take to reinforce the alliance’s nuclear posture and readiness in the absence of a more visible presence of U.S. assets.

5. It is not clear the necessary processes and structures are in place to make difficult — and informed — prioritization decisions.

Prioritization of the nation’s resources to account for the possibility of opportunistic aggression is needed. Not addressing U.S. force posture to accommodate the most likely opportunistic aggression scenarios could make such scenarios more likely due to an adversary perceiving a weakness in U.S. military strength. The U.S. defense industrial base — both in terms of its production capacity and workforce — likely cannot quickly accommodate major changes in U.S. military force posture to account for the risk of opportunistic aggression. The U.S. military’s budget is stretched in many ways, and significant adjustments have proved difficult in the past and will likely be difficult in the future. These challenges point to the need for prioritization when considering how to mobilize the defense industrial base to address this problem. But it is not clear the necessary processes and decisionmaking structures are in place across services and departments for making informed resourcing decisions — not just for the nuclear enterprise but also across the broader defense industrial base. Congress and the executive branch need to address this gap.

Recommendations

1. Review intelligence priorities.

Understanding when a leader might choose to initiate aggression or use nuclear weapons — and what specifically might deter them from doing so — is already one of the most difficult intelligence priorities. Assessing how a leader’s calculations might shift if the United States and its allies were already engaged in a conflict elsewhere complicates this matter significantly. Policymakers should review current intelligence priorities to ensure the intelligence community is also considering what factors might influence the likelihood a state will opportunistically initiate aggression while the United States is already involved in a conflict. Improving collection on and assessment of these dynamics not only will allow the United States and its allies to better anticipate when opportunistic aggression might occur but will also provide a critical baseline for better tailoring deterrence strategies and understanding how those strategies might interact.

2. Establish a Nuclear Policy Council.

The task force recommends creating a senior interagency group with statutory authorities to more consistently and over longer periods review deterrence policy and ensure planning is congruent with nuclear policymaking in a dynamic threat environment. This group would serve as the interagency policy counterpart to the existing DOD-NNSA Nuclear Weapons Council (NWC), which oversees the U.S. nuclear weapons stockpile management process and ensures alignment between DOD delivery systems and NNSA warheads. The new group would serve as a bridge between the NWC and higher-level National Security Council–led policy discussions. Among the many benefits of such a standing group, continuity is the most crucial advantage. Rather than wait for the NPR process to take place every several years, a policy-focused council would ensure ongoing long-term analysis without undercutting an administration’s ability to conduct independent reviews at its discretion. The council could also more closely track operational considerations and how policy translates to planning, resourcing, and programming decisions, and vice versa.

3. Prioritize war games, TTXs, and scenario-based discussions that focus on opportunistic aggression.

While the DOD conducts numerous war games each year, the vast majority of them are limited to conventional operations. To further complicate the issue, the DOD does not maintain an annual war game in which an adversary employs limited nuclear weapons or in which a second adversary launches an attack during an existing regional conflict. Limited nuclear use and opportunistic aggression scenarios are often addressed only in localized exploratory TTXs, resulting in shortfalls across the DOD in understanding theater-level strategy and escalation management approaches for scenarios in which an adversary uses limited nuclear attacks or U.S. forces must also contend with opportunistic aggression from a nuclear adversary.

War games and TTXs that focus on a range of opportunistic aggression scenarios could help senior leaders, combatant commands, and planners understand challenges and resource constraints, as well as force management limitations. However, these cannot just be games led and attended by USSTRATCOM; interagency and senior administration officials outside of the DOD should also be included. This can help senior leaders feel more comfortable making decisions in resource- constrained environments and better understand the trade-offs and potential opportunity costs of certain capability investments. Running through scenarios with allies and diversifying the participant base can also improve understanding of the potential constraints. Unclassified games outside of official channels can also help increase the overall understanding of these challenges and provide a platform for engaging key allies on these issues.

4. Develop an engagement plan for allies to empower more detailed and transparent conversations on opportunistic aggression.

The United States needs to develop an engagement plan for deepening planning and consultations with allies on opportunistic aggression scenarios. Opportunistic aggression scenarios need to be on the agenda with allies in existing dialogue structures. These could be added as an agenda item or incorporated into scenario-based discussions or TTXs.

Open and honest conversations with allies could improve understanding of what might be asked of them in certain scenarios. To do this effectively, the United States must take the lead on normalizing theoretical conversations with allies while managing perceptions of assurances. Hypothetical conversations will enable allies to better understand the threat environment, limitations on U.S. capabilities, and the kind of burden sharing that will be expected or necessary. This discussion could occur via existing mechanisms and channels, or it could be a wider effort that the DOD and the State Department co-lead.

5. Prioritize nuclear deterrence education for senior leadership.

The DOD does not sufficiently educate Joint Force leaders on nuclear issues. This gap in knowledge stems from a lack of standardization and mandatory instruction on nuclear issues across professional military education (PME). Many Joint Force officers receive little to no exposure to nuclear issues, including effects, policy, deterrence theory, escalation management, and opportunistic aggression, unless taken as an elective. In fact, when a leader becomes a senior flag officer, it may be the first time they are exposed to nuclear planning and operational concepts. This needs to change. Officers need exposure to these issues much earlier in their careers.

Policymakers and planners also need deeper expertise on both nuclear issues and planning for conventional conflicts. In the diplomatic realm, policymakers leading consultations and conversations with allies often lack detailed understanding of nuclear (and conventional) planning and operational concepts, making it difficult to have frank conversations about what might be needed, by whom, and when. Further, senior leaders and political appointees need better exposure to how integrated planning, including nuclear planning, is undertaken.

6. Diversify U.S. nuclear forces.

U.S. policymakers should strive to diversify U.S. nuclear forces through investments in new capabilities, possibly in smaller numbers. This would provide the president with a broader range of credible options, particularly if an adversary threatens limited nuclear attacks. Strategic deterrence is, and should remain, the primary mission of the U.S. nuclear force, and the triad is essential to the success of that mission. Nonetheless, nuclear capabilities with the following attributes will likely contribute significantly to U.S. ability to deter opportunistic aggression prior to it occurring in a second theater, or aid in restoring deterrence should it fail or if limited nuclear use occurs:

• survivability (ability to penetrate enemy air defenses and maneuver)

• persistent presence

• lower yield

• responsive and effective across a spectrum of targets

These capabilities should also serve certain objectives, such as the following:

• regional assurance

• operational flexibility

• deterrence (both prior to conflict and within it)

The task force concludes that sea-launched weapons with these attributes are likely the best option to meet the opportunistic aggression threat posed by China in the Indo-Pacific and may also serve other roles in a European theater depending on the circumstances.

7. Address critical conventional shortfalls.

The risk of opportunistic aggression is increased by conventional shortfalls in key capabilities, including long-range fires, precision-guided munitions, and heavy airlift and refueling. While current stockpiles may be sufficient to achieve victory in a single brief war, the war in Ukraine demonstrates the difficult task of replacing expended munitions and destroyed platforms in sustained high-intensity conflict. The United States and its allies must acquire enough of these key capabilities to fight and win one war while maintaining a sufficient conventional deterrent to prevent a second. To do so, officials should solidify a clear division of labor and investment strategies with allies. Building the defense industrial base and workforce capacity both in the United States and within allied nations to support shifting force structure plans will be key. The United States must rely on allied nations to fill the conventional weapons gaps they are capable of filling, such as lower-end munitions to support the war in Ukraine. That is not to say that allied nations should always have the role of low-end munitions production. But this is a near-term ask, and division of labor can help fill key gaps.

8. Accept calculated risks.

A viable strategy for addressing the four resource challenges posed by opportunistic aggression, previously described in this report, is not to simply build more but to build different. The key decision here is determining how the United States is committing itself to engage in a simultaneous two-conflict scenario and where it is willing to accept risk. U.S. nuclear modernization efforts are largely focused on one-for-one replacement programs, which will put new versions of extant weapons in the triad into service. The main lines of effort of the nuclear modernization program should continue as planned, but the authors advocate for increasing diversity in the modernized triad, in terms of both of baseline and regional deterrence capabilities. This may drive hard decisions to reprioritize some modernization programs that offer the same deterrent benefit as the weapons they are replacing. The United States should be willing to accept calculated risks in limited circumstances to free up resources for the development and fielding of systems that offer greater nuclear force diversity. This requires improved processes and structures to make difficult — and informed — prioritization decisions across both the nuclear enterprise and the broader defense industrial base.

Policymakers should also shift their cultural mindsets and become more risk tolerant — from both a technological perspective and a political perspective — and allow the advanced development communities at DOD laboratories and NNSA national laboratories to experiment more aggressively. This means lowering the barrier to starting and canceling new weapons programs. The national laboratories should be encouraged to experiment and explore various options for the U.S. nuclear stockpile, even when those experiments do not lead to an established weapon in the stockpile every time. Given the trajectory of the global security environment, new technologies could provide policymakers with more flexible options to meet U.S. national objectives — if U.S. scientific and technical communities are permitted to do so. Policymakers should be willing to accept calculated risks to expand opportunities, including the risk of capability failure. The ability to more easily start and cancel programs and embrace truly flexible acquisition strategies would allow U.S. nuclear posture to better adjust to military needs and be more responsive to both positive and negative changes in the security environment.

David M. Allison is a political scientist with a focus on the intersection of emerging technology and strategic stability. He is a lecturer at Yale University and an associate with the Project on Managing the Atom at the Belfer Center for Science and International Affairs, Harvard Kennedy School.

Savannah Blalock is a foreign affairs specialist in the Office of Nonproliferation and Arms Control at the U.S. Department of Energy’s National Nuclear Security Administration (NNSA).

Micah Howard is an operations research analyst and the nuclear forces portfolio lead in the Office of the Secretary of Defense (OSD), Cost Assessment and Program Evaluation (CAPE).

Tim McDonnell is a member of the research staff at the Center for Naval Analyses. His primary area of expertise is in nuclear weapons policy, including nuclear strategy, deterrence and extended deterrence, arms control, and the relationship between nuclear posture and foreign policy.

E. Paige Price is an assistant professor of strategy and security studies at the School of Advanced Air and Space Studies (SAASS). Her research focuses on issues of international security, nuclear security and nonproliferation, and strategic deterrence.

Victoria Sanchez is a foreign affairs specialist at the U.S. Department of Energy’s National Nuclear Security Administration. During the work of the task force, Dr. Sanchez was at the U.S. Department of State as a foreign affairs officer in the Bureau of European and Eurasian Affairs Office of Policy and Regional Affairs.

Stephan Varga is a lieutenant colonel in the United States Army and serves as FA52 nuclear and counterproliferation officer in the Pentagon. He has over 20 years of experience in combating weapons of mass destruction, nuclear weapons effects, modeling, and policy.

Angela Weaver currently serves as the section head for strategy and policy at Navy Strategic Systems Programs, where she is responsible for all Reentry Branch external policy coordination and Navy nuclear deterrence mission support.

Kelsey Hartigan is the deputy director of PONI and a senior fellow with the International Security Program (ISP) at CSIS.

Reja Younis is the associate fellow with PONI in ISP at CSIS, she leads research on nuclear deterrence issues, nuclear strategy, and emerging technologies.

Lachlan MacKenzie is a program coordinator and research assistant with PONI in the International Security Program at CSIS.