At a meeting earlier this year, top foreign ministry officials from several European countries were asked who among them still believed that the United States would come to their defense if they were attacked. Not a single hand went up. It was a stark indication of the extent to which European governments have concluded that the United States—for so long the backbone of European and NATO security—is no longer a reliable ally. Over the past 18 months, conversations in Brussels and in other NATO capitals have revolved around how to boost the continent’s conventional defense to make up for Washington’s retrenchment. But a quieter and more urgent discussion is also taking place, focused on the status of the ultimate deterrent: the U.S. nuclear guarantee.

More than half a century ago, the Great Debate, as the French strategist Raymond Aron termed it, broke out between European countries and the United States over how to preserve European nuclear deterrence in the face of the Soviet Union’s expanding nuclear capabilities. Then, European leaders feared that Washington’s growing emphasis on the conventional defense of the continent reflected an increasing reluctance to protect Europe with nuclear weapons—especially as Moscow developed missiles that could reach the U.S. homeland. Would the United States be willing to risk Boston to defend Berlin? That debate was ultimately resolved through a new grand bargain: in return for a commitment from European countries to forgo developing nuclear weapons, the United States would invite more European participation in nuclear decision-making. That essential bargain stuck for 60 years, helping secure both the United States and Europe and preventing nuclear proliferation.

Now, however, that bargain has come undone. Washington says that it wants a new division of labor: the Europeans need to take on primary responsibility for their conventional defense while the United States continues to ensure their nuclear defense. But that hardly addresses the problem. “If you don’t believe in the United States’ Article 5 commitment,” one of the officials attending the meeting told me, “then extended deterrence is effectively dead.” Already, alternative options are being considered. In some European capitals, officials are quietly raising the possibility of acquiring their own nuclear weapons. France, meanwhile, has offered to adapt its nuclear policy to help defend its European allies.Each of these options has real logic. But none of them adequately addresses the problem of ensuring European security without risking further proliferation. A better approach would be a fourth alternative: a renewed U.S.-European nuclear bargain, this time with a real European core. Led by France and the United Kingdom, NATO’s European members need to take on a larger role in their nuclear deterrence while working to restore the transatlantic defense link that has been the foundation of U.S. and European security for so many decades.

The Soviet Union’s 1957 launch of the Sputnik satellite posed a profound dilemma for NATO. Until then, the alliance had counted on U.S. nuclear weapons deployed in Europe to deter an attack by a much larger Soviet conventional force. But once Moscow demonstrated its ability to launch intercontinental rockets, Europeans feared that the United States would no longer be willing to use nuclear weapons to prevent a Soviet invasion of Europe.

These concerns prompted a fierce debate within NATO about how the continent could ensure its nuclear defense. Some, led by France, were convinced that European countries needed their own nuclear weapons. Others sought less costly ways to restore some control over nuclear deterrence. One American idea that gained substantial traction was to create a so-called multilateral force—a fleet of vessels carrying some 200 missiles. Such a multilaterally operated and funded force could give Europeans a sense of ownership and control over the use of nuclear weapons, the thinking went, while preserving the U.S. president’s final authority over their launch.

The debate exposed a fundamental problem with extending U.S. nuclear deterrence to allies, especially if an ocean separates those allies from Washington. An adversary might be deterred from attacking an American ally by even the small possibility of a U.S. nuclear retaliation; the consequences of a miscalculation would be catastrophic. But that same small possibility would offer little comfort to the ally, who needs to believe that Washington would actually risk nuclear devastation on its behalf. Denis Healey, the United Kingdom’s defense secretary throughout much of the 1960s, postulated what became known as the Healey theorem: “It takes only a five percent credibility of American retaliation to deter an attack, but it takes a 95 percent credibility to reassure the allies.”

The Sputnik launch posed a profound dilemma for NATO.

The debate was ultimately resolved through a bargain. Washington’s European allies agreed to forgo their right to acquire nuclear weapons in return for a greater say in U.S. nuclear weapons policy and strategy. Although the idea of a costly multilateral force was abandoned, the creation of the NATO Nuclear Planning Group (NPG) provided a vehicle for Europeans to give input into U.S. nuclear........

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