To understand why, we need to go back to 1963, when President John F. Kennedy warned of a “world in which 15 or 20 or 25 nations may have these weapons.” Kennedy expressed the widely held belief that further proliferation was likely, if not inevitable. Every nation that possessed the capability to build a bomb had done so and American officials worried that the trend was about to accelerate.

That didn’t happen. Having stood at the brink of nuclear war during the Cuban missile crisis in 1962, the United States and the Soviet Union redoubled efforts to stabilize their nuclear relationship and prevent other states from crossing the nuclear threshold. The nonproliferation treaty was one result of those efforts. Under the treaty, states that didn’t have nuclear weapons pledged not to develop or acquire them, while those that did committed to eventual nuclear disarmament.

But it wasn’t just U.S.-Soviet arms control negotiations that turned the proliferation tide in the 1960s. Even more important was Washington’s determination to assure its allies in Europe and Asia that they could rely on America for their nuclear security. Only after they were convinced that the American nuclear guarantee was credible, did allies like Germany and Japan decide to forego a national nuclear option and join the nonproliferation treaty.

Whenever new developments seemed to call the American guarantee into question — as when a new generation of Soviet medium-range missiles were deployed in Europe in the 1970s and when North Korea expanded its nuclear and missile programs in the 1990s and 2000s — Washington worked to reassure its allies that its nuclear commitment remained strong and credible.

In recent years, new questions about the credibility of the American nuclear guarantee have returned. One reason is the changing strategic environment. In Europe, a more adventurous and better-armed Russia no longer shies away from using military force, as its invasion of Ukraine and its support for the Assad regime in Syria have underscored. In Asia, China’s rapid rise has expanded its military reach throughout the Asia-Pacific, and North Korea has emerged as a potent foe, armed with nuclear weapons and long-range missiles that threaten the entire region.