The United States and its allies in the Indo–Pacific, Europe, and the Middle East face a security challenge beyond anything they have seen before. In addition to displaying newfound confidence, autocrats in Beijing, Moscow, Tehran, and Pyongyang are increasingly threatening the United States and its allies and increasingly carrying out acts of overt aggression.[REF]

At the same time, the scope and scale of the threats facing the United States and its allies affect both theater nuclear deterrence dynamics and, increasingly, central nuclear deterrence dynamics with potential impact for the American homeland. Indeed, China, Russia, and North Korea are building up their strategic and theater nuclear arsenals, undoubtedly for purposes of deterring strategic attack against their homelands, but likely also for purposes of nuclear warfighting.[REF]

In the current security environment, theater nuclear war is the more likely scenario for nuclear employment.[REF] Nevertheless, the possibility of central deterrence failure that results in a nuclear exchange between the American homeland and a nuclear peer cannot be ruled out. Such an exchange could result from a theater nuclear war that escalated to a central nuclear war or due to a direct conventional war between the United States and a nuclear peer.[REF] In short, the nuclear expansion of the autocrats, combined with their newfound confidence, proclivity for aggression, the relative weakness of American allies, and adversary conventional capabilities, makes the chance of deterrence failure a real possibility in the coming years.[REF]

All this comes at the worst time for the United States. The current military does not have a conventional force sized for a two-war strategy to stop opportunistic or coordinated aggression in two geographically distant theaters.[REF] In addition, the U.S. nuclear force is old and sized for a more benign world that never materialized.[REF] The current U.S. nuclear modernization is therefore necessary, but insufficient for the current conventional and nuclear threats posed by America’s adversaries.[REF]

The findings of the 2023 Strategic Posture Commission as they pertain to right-sizing the U.S. nuclear arsenal are generally correct.[REF] Certainly, a more diverse and capable range of nuclear options is essential to U.S. and allied security—particularly as it relates to deterring aggression or perhaps more critically, deterring adversary nuclear employment. Indeed, a more diverse set of nuclear options—to include weapons that have a variety of characteristics, with a variety of ranges, that could be delivered across multiple domains—strengthens U.S. deterrence posture, the credibility of U.S. extended deterrence, and achieving objectives should deterrence fail, particularly if U.S. conventional forces are critically engaged in combat operations in other theaters.[REF]

For these reasons, the United States must look critically at fielding a somewhat larger, more diverse, nuclear strike force in order to strengthen its ability to support both regional and central deterrence missions.

Strengthening the Shield and the Sword

It would, however, be foolish to respond to the gathering threats by only making changes to U.S. nuclear strike forces. Indeed, the most effective deterrence strategies involve capabilities to reduce the effectiveness of an adversary’s potential attack and to respond to the attack by inflicting unacceptable costs on the adversary.[REF] If an adversary fails to achieve its objectives in an attack and provokes an unacceptably costly U.S. response, then that adversary would have the worst of both worlds: paying a very high price to achieve little, if any, gain. Put another way, by denying an adversary the benefits of an attack through the fielding of effective defenses, coupled with a credible capability to inflict significant damage on the attacker, the United States can create a credible deterrent posture that rejects mutual vulnerability and ultimately dissuades an adversary from taking aggressive action to begin with. Such a strengthened deterrence posture—one that better integrates nuclear-strike capabilities with theater and more directly with homeland air and missile defenses—is an imperative. This is particularly true given that the stakes for deterrence failure in the emerging two-nuclear-peer threat environment are simply too great to dismiss the deterrent value of U.S. integrated air and missile defenses, both in the American homeland and in overseas theaters.

If the United States builds more and better nuclear weapons, as this author and many others have advocated,[REF] adversaries are likely to find U.S. deterrence threats more credible. But the real gains in credibility will come from adversaries believing the United States has a clear and ideally decisive defensive advantage during a conflict. As Fred Hoffman said in defense of President Ronald Reagan’s 1980s Strategic Defense Initiative, a credible threat is one that the adversary sees “as serving Western interests in the event of an actual crisis or conflict.”[REF]

In this sense, a credible and potentially decisive defensive advantage during an exchange between two nuclear peers influences both actors’ behavior—but it almost certainly has greater influence on the actor that lacks strategic defenses, but that nevertheless may be inclined to initiate conflict due to its status as a revisionist state (as China, Russia, Iran, and North Korea are today). Put another way, it is one thing to gamble on a roll of the dice where either side in a two-nuclear-peer conflict might prove more willing to endure homeland-to-homeland coercive nuclear strikes—but it is a whole other game entirely when the dice are weighted in favor of the actor that fields credible strategic defenses. Thus, if an adversary believes the United States can substantially blunt its attack and is unable to do the same to the inevitable U.S. response, it is far less likely to escalate a conventional conflict to the nuclear threshold, and, indeed, less likely to initiate a conflict—even a conventional one—in the first place.

Moreover, the United States can never hope to have perfect control over a conflict. History is replete with examples in which the United States hoped to avoid a conflict, only to get dragged into it.[REF] History is also full of examples in which a conflict begins under the assumption that American involvement would remain limited, only to see said involvement intensify and become protracted.[REF] Perfect control is therefore not possible—and nor is it the goal of improved and expanded air and missile defenses. Rather, the United States can and should appear to adversaries as more capable and therefore more likely to impose restraint on them through credible homeland air and missile defenses than U.S. adversaries are willing or able to impose on it, due to their lack of similar defenses. As Herman Kahn said, “Usually the most convincing way to look willing is to be willing.”[REF]

The best way to look willing to defend allies in a theater conflict is to have a credible theory for limiting damage from an adversary attack. U.S. missile defenses, both homeland and theater, rightly should be viewed as stabilizing against nuclear use in a theater conflict. Said defenses, if credible, lower the chance that an opponent will conclude that the United States cannot or will not respond to limited theater nuclear use to redress failed conventional aggression for fear of subsequent limited nuclear attack against a vulnerable homeland. More important, it lowers the chance that the opponent attempts the even better strategy of limited nuclear attack against a regional U.S. ally in an effort to break U.S.-led alliances or change the operational trajectory of a conflict. Such an approach is not new. It in many ways replicates the Cold War–era “trading New York for Paris” dilemma.

With credible theater or strategic missile defenses, opponents are forced to “go big” in their nuclear attack, whether such an attack be against targets within a theater or against the U.S. homeland, to achieve the same effect of a limited nuclear strike. By fielding credible defenses that cause the adversary to face the prospect of launching a sizable nuclear attack, only to achieve a fraction of the desired effect, and then facing the prospect of a vigorous nuclear response from the United States, Washington raises the chances that the adversary will choose to “not go at all.”

Indeed, not only would credible theater and strategic missile defense make the United States look better prepared to resist coercion by limiting the damage an adversary could inflict, it also makes the United States look better prepared to respond to an attack decisively and thereby impose costs on the adversary. This perception of strength is almost as important as the actuality of strength. As is known to everyone who has ever attended middle school, bullies do not target the strong, only the weak—and a largely defenseless United States may give adversaries the impression that if they aim their attack just right, the United States will be deterred from responding forcefully, or at all. In short, the point of fielding credible missile defenses is not to move into a defense-dominated strategy, but to avoid an “excessive reliance on offensive forces” while still maintaining strength.[REF]

To summarize: Advanced and expanded theater and homeland air and missile defenses are to convince adversaries that nuclear coercion or limited nuclear attack against the United States or an ally is unlikely to work—and that U.S. leadership may be more willing to employ nuclear weapons in extreme circumstances (such as the aforementioned attempts at nuclear coercion) if its homeland and overseas forces and allies are protected by credible defenses.

Enduring Myths: Missile Defenses Are “Destabilizing” and Trigger an “Arms Race”

Critics of expanded and improved U.S. homeland air and missile defenses often claim that building such defenses will be “destabilizing”—a condition that is more often asserted than examined.[REF] If “stabilizing” is defined as a condition in which neither side has an incentive to employ nuclear weapons against the other first, then air and missile defenses fit the definition of “stabilizing” perfectly because, as noted, credible air and missile defenses makes an adversary strike less likely to be effective.

Nevertheless, critics of missile defense, particularly homeland missile defense, typically assert that missile defense is destabilizing because it will cause an arms race by triggering adversaries to build more long-range missiles as a means to overcome missile defenses or pursue other capabilities that can circumvent missile defenses.[REF] According to this line of argument, such an expansion of missile arsenals will exacerbate political tensions and could, ultimately, lead to war.[REF] Despite the fact that such critics rarely define what an “arms race” is,[REF] history does not support the contention that homeland missile defenses trigger arms races. This was not the case after the United States withdrew from the Anti-Ballistic Missile Treaty more than 20 years ago and there is little evidence that Russia or even China will accelerate the deployment of strategic weapons after President Donald Trump’s announcement to build the Golden Dome missile defense architecture.

Further, there is no historical example of unilateral U.S. restraint, in either offensive strike capabilities or defensive capabilities, resulting in reciprocal restraint by any adversary absent a formal agreement to do so. Indeed, U.S. reduction in operationally deployed nuclear weapons preceded the Soviet Union’s continued build-up in its own nuclear stockpile, thereby triggering U.S. Secretary of Defense Harold Brown (1977–1981) to famously say: “When we build, they build, when we stop, they build.”[REF] This truth is still applicable today, but with the difference being that the United States now has two nuclear-armed adversaries building up.

However, even if it were true that China and Russia are certain to respond to the deployment of credible homeland missile defenses by accelerating their fielding of an even greater number of long-range missiles, critics should admit that U.S. restraint in missile defense before the announcement of Golden Dome was no solution to adversary missile proliferation, given that Russia and China have been increasing the number of deployed long-range missiles for years. In addition, these adversaries would have to consider what a launch of significant numbers of long-range missiles at the American homeland in sufficient quantity to overcome U.S. homeland missile defense (even if part of a limited coercive strike) would look like to leaders in Washington.

Beyond arms racing, those opposed to homeland missile defense extend their criticism to other areas. For them, the most destabilizing security dynamic is where one party has sufficient offensive capability relative to the opponent such that it can strike the opponent first with confidence that the opponent’s remaining capability is less than the striker’s defensive capability, combined with its own tolerance for any damage inflicted upon the striker.[REF] Within this line of thought, the more defensive capability one party has, the more likely an actor will be to strike an opponent first—hence the assertion that defenses are “destabilizing.” Taken to the logical conclusion, this is the basis for the assertion that “mutual vulnerability” is the ultimate stable condition.[REF]

Of course, while this criticism may—may—work in theory, such stability by means of mutual vulnerability is predicated upon both actors being willing to allow their own nuclear-armed opponents a say in whether their nation will continue to survive. Mutual vulnerability is, at its core, an agreement among multiple parties to hold each other’s national existence hostage. While such arrangements may work over a period of time between two parties if both actors place a similar value on the existence of their own national survival or the survival of their populations, does such an arrangement still work if more than two parties are engaged? Does it still work if multiple parties are deeply hostile to a single party, say, a group of nuclear-armed, deeply revisionist autocratic actors who field combined nuclear arsenals larger than that of the liberal democratic, status-quo actor? One would hope that testing this hypothesis in a real-world environment can be avoided.

Moreover, there is no evidence that an actor who enjoys significant homeland defenses and nuclear-strike options in relation to an opponent’s nuclear options would in fact be more likely to carry out a first strike against an opponent. In fact, for the first 15 years of the Cold War, when the United States had qualitative and quantitative nuclear advantage over the Soviet Union and could in fact carry out an exquisite first strike against the Soviets, the United States chose not to engage in a first strike against the Soviet Union.[REF] Nor did it strike China in the early 1960s when it became clear that China was pursuing nuclear weapons. Consequently, the burden of proof should be on the critics of homeland missile defense to show that mutual vulnerability is a highly stable (and therefore, desirable) security arrangement in a world of revisionist, authoritarian nuclear peers, and that credible missile defenses are destabilizing.

This Backgrounder posits that in a three-nuclear-peer world, mutual vulnerability is no longer applicable, as the combination of any two against one can set up a scenario where together two actors can achieve first-strike superiority even if individually they cannot. In such a world, mutual vulnerability is highly destabilizing as it gives two actors an incentive to perhaps try to execute an exquisite first strike, even outside a high-intensity conflict. In such a three-nuclear-peer world, the ultimate stabilizing factor is a secure second-strike capability that will likely require active homeland missile defenses capable of maintaining credible defenses against the threat of two on one.

If the United States had few or no homeland missile defenses in such a world, then nuclear-armed peers that wanted to strike a handful of targets in the U.S. homeland would need to fire only a handful of missiles. Such a limited strike could cause the United States to respond with overwhelming force, but may instead cause the United States to respond in kind or seek a negotiated settlement to the conflict after a proportional response—but it is unlikely that Washington would overreact to such a limited strike.

If, however, the United States had significant defenses, and adversaries still sought to attack a handful of targets in the U.S. homeland to send a coercive message, then they likely would have to launch dozens to hundreds of missiles to overcome a credible homeland missile defense architecture. Such an act would be far more likely to trigger a U.S. response that is unacceptable to the attacker. In such a scenario, there would be almost no chance of leaders in Washington willing to seek any kind of negotiated settlement or respond with a limited strike.

As noted, some critics of homeland missile defense point to Russian and Chinese complaints about the “destabilizing” nature of U.S. homeland missile defense,[REF] regardless of the sincerity of such complaints. Surely, some Chinese and Russian complaints about the “destabilizing” nature of U.S. homeland missile defense are partly for show, and partly because China and Russia themselves are building their own integrated air and missile defense networks.[REF]

It is also important to remember that an arms race is not assured, as adversaries face resource prioritization decisions, just as does the United States. Does Russia want to spend more on building long-range missiles, warheads, countermeasures, and more to defeat U.S. homeland missile defenses? Or does Moscow want to invest those funds in rebuilding its conventional forces following significant material losses in the Ukraine War? Similarly, does China want to spend more funds to defeat U.S. homeland missile defenses? The more money spent on overcoming U.S. missile defenses means less money spent on building the conventional forces necessary for taking Taiwan or for policing its autocratic-ruled society.

Conclusion

The United States needs more and better integrated air and missile defenses, protecting both allies and U.S. forces in overseas theaters and the U.S. homeland, due in part to the United States’ role as an offshore balancer in defense of its allies and partners around the world. When the U.S. homeland is threatened, and U.S. ability to project power is diminished, then the entire U.S. defense strategy falls apart—and adversaries will be further emboldened to strike at the existing global order. All of this goes against U.S. national interests.

In the coming years, with the United States committed to defending allies in multiple theaters against multiple adversaries, these adversaries will have additional incentives for aggression. U.S. credibility will be tested and America cannot afford to be seen as a paper tiger. More conventional capabilities—to include ships, planes, munitions—will be needed in such a world.

But improved and expanded theater and missile defenses, coupled with larger and more diverse credible offensive nuclear-strike capabilities, are the most stabilizing posture the United States can field at this time. Such capabilities deter adversaries and assure allies, and, most important, limit damage to the U.S. homeland should deterrence fail.

To this end, the United States should abandon, or at a minimum, stop genuflecting at the altar of “mutual vulnerability.” Not only is mutual vulnerability no longer an optimal strategy in a three-peer world (if it ever was a good strategy in a two-peer world), but the United States must remember the obvious, but too-often-unstated, point that Russia and China simply do not accept the Western concept of mutual vulnerability being the highest goal. As the CIA wrote nearly four decades ago, “the Soviets have maintained the more traditional military view that forces prepared to fight a war are also better able to deter war; they have never subscribed to Western concepts, such as Mutual Assured Destruction, that draw sharp distinctions between the strategic force requirements for deterring a nuclear war and those for fighting one.”[REF] (Emphasis added.)

Finally, the coming years may lead to a scenario where any party that has a secure second-strike capability while in a “two-on-one” scenario and that has also achieved a first-strike superiority against one or both of the other two actors may be perceived as a destabilizing and highly threatening actor by one or both of the other two (comparatively) weaker actors. Hence, one could conclude that strategic stability in a three-party world cannot be achieved from weapon systems characteristics alone. It may require political actions. Such a world may be possible in the coming decade.

In the final analysis, the U.S. government is ultimately responsible for two things: protecting the American people and being prepared to defeat America’s adversaries. U.S. policymakers should therefore think seriously and deeply about what is required to defend the American people and ensure that the United States fields sufficient secure second-strike capabilities to deter two nuclear peers simultaneously.

To achieve all this, it is imperative that the United States field more, and more diverse, offensive nuclear-strike options, to include those based in the homeland and those forward deployed in various theaters. This will require a nuclear enterprise—particularly, a reinvigorated National Nuclear Security Administration—that can build new warheads at scale and repurpose old warheads from the strategic stockpile for use in overseas theaters. This also will require significant investment in expanding the number of theater air and missile defense options as well as a significantly expanded homeland missile defense architecture, such as Golden Dome. None of this will be cheap—but deterrence failure would be far more costly.

Robert Peters is Senior Research Fellow for Strategic Deterrence in the Douglas and Sarah Allison Center for National Security at The Heritage Foundation.