For decades, American allies in Asia and Europe have relied on U.S. extended nuclear deterrence for their safety. They forswore acquiring their own nuclear weapons, agreeing instead to live under the protection of the United States’ nuclear umbrella. This arrangement worked during the Cold War because the stakes of that competition were so high that the United States could credibly—if just barely—say that it would wage nuclear war, even at catastrophic risk to itself, to prevent the conquest of its allies.

But the world that created the U.S. extended deterrence system is long gone. The United States retains important interests in Europe, but Russia’s threat is far more limited than when the Soviet Union was poised to conquer much of Europe and Asia. A war today would be about the precise boundaries of Moscow’s influence in eastern Europe, not the global balance of power. Similarly, the Korean Peninsula was once a battleground for rival Cold War blocs; today, that conflict is largely local, with Pyongyang desperately seeking to hold on to its power. And the Trump administration’s view that the purpose of alliances is first and foremost to enhance the United States’ core national interests is at odds with promises to jeopardize American cities to defend allies. In a world in which wars are largely regional and Washington is pulling back, it strains credulity to believe that the United States would wage nuclear war for faraway allies.

U.S. allies are thus increasingly worried that the extended deterrence system is now a bluff. Their fears are justified. Leaders in Latvia, Poland, and South Korea cannot be confident that the United States would risk its own destruction for their safety. Those in North Korea and Russia may be drawing the same conclusion. If Washington’s nuclear commitments are no longer credible, U.S. allies need new arrangements to ensure their most critical security needs.

The crisis of extended nuclear deterrence has developed gradually since the end of the Cold War. When allies have questioned the credibility of American promises, Washington has sought to soothe their concerns. The United States issues official statements declaring its commitment to its partners and conducts military demonstrations, such as submarine port visits or bomber overflights, to underscore U.S. power and commitment. But it’s dangerous to try to convince allies that all is well when it’s obviously not. Refusing to address the weakness of the existing extended deterrence strategy could embolden adversaries to attack. And fearing for their safety, allies in Asia and Europe could rush to cobble together their own nuclear programs, which could in turn trigger preventive strikes and the rapid escalation of conflict.

The United States must accept that it can no longer be a credible nuclear guarantor for all of its allies around the world. It needs to get comfortable with its partners seeking solutions more suited for today’s geopolitical realities, including, in some cases, acquiring their own nuclear weapons. In Asia, this could mean establishing a nuclear sharing agreement with South Korea, modeled on the one between Washington and NATO, in which the United States would promise to transfer control of tactical nuclear weapons to Seoul if North Korea were to attack. Or if Seoul ultimately chooses to withdraw from the Nuclear Nonproliferation Treaty to build its own nuclear arsenal, Washington should support this decision and use its influence to insulate South Korea from the dangers of such a transition. In Europe, the best option for the United States and its partners is to modify the existing NATO nuclear sharing program to increase European control over the alliance’s collective deterrent. France and the United Kingdom—the two other nuclear-armed NATO states—could enhance their role in NATO’s nuclear plans to reduce Europe’s dependence on Washington and better protect the bloc’s most vulnerable members.

Helping U.S. partners develop stronger and more autonomous deterrence postures is anathema to many in Washington, who fear nuclear proliferation and do not want to give up the United States’ leverage as a global security provider. But this may not be Washington’s decision. U.S. allies are vulnerable, and they are right to take concrete steps to reduce the real dangers they face, whether that means developing their own nuclear deterrent or making other moves that violate outdated U.S. prohibitions. Washington should support them along whichever path they choose for protection.

CREDIBLE ARRANGEMENTS

The U.S. extended deterrence strategy grew out of two conflicting Cold War imperatives. The United States recognized that nuclear weapons were essential for preventing Soviet attacks against American allies around the globe. Yet Washington and its partners did not want scores of countries to have their own weapons because they feared a world of competing nuclear-armed states. The solution was a system of extended nuclear deterrence in which the United States opened its nuclear umbrella over its most important allies in Asia and Europe. Although France and the United Kingdom decided to acquire their own arsenals, American nuclear weapons served as the strategic deterrent for the rest of the U.S. alliance system during and after the Cold War.

From its inception, this strategy had credibility problems because it seemed far-fetched that the United States would risk nuclear catastrophe to defend its allies. Indeed, in 1961, French President Charles de Gaulle scoffed at the notion that the United States would sacrifice New York City to defend Paris. If anything, de Gaulle greatly understated the extent of the challenge: U.S. policy effectively required Washington to put every major American city at risk to protect its allies.

To enhance the credibility of extended deterrence, the United States devised an array of military actions and diplomatic maneuvers. Washington deployed troops and tactical nuclear weapons near the border between East and West Germany and just south of the dividing line between North and South Korea. By stationing nuclear weapons directly in the path of a potential invasion, where they would surely become entangled in any fighting, the United States made it clear that a major attack risked catastrophic escalation.

Leaders of both U.S. political parties buttressed those deployments with rhetoric that signaled the inseparability of American and allied interests. In 1963, when President John F. Kennedy, a Democrat, stood in Berlin and declared, “Ich bin ein Berliner,” he was emphasizing that Washington and its key NATO allies were one. More than two decades later, President Ronald Reagan, a Republican, struck the same note when he told........

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