Given contradictions like this, it can be hard to take Trump seriously on foreign policy. But the implications of what he has said on nuclear weapons are extremely serious. A Trump presidency could reverse decades of American presidents’ work to hold the line against the spread of nuclear weapons, ushering in a new era of proliferation. U.S. leaders have applied “tremendous pressure” on allies to get them to turn back their nuclear programs. They have led efforts to successfully reduce the number of states that had or were actively pursuing nuclear weapons, from 23 in the 1960s down to nine.

At the core of Trump’s proliferation “policy” is a mistaken, reflexive belief that America is weak and will be powerless to stop the spread of nuclear weapons. “[I]f the United States keeps on its path, its current path of weakness,” he told The New York Times in March, Japanese and South Korean leaders would want such weapons “with or without me discussing it, because I don’t think they feel very secure in what’s going on with our country.” To CNN, he said: “It’s going to happen anyway. It’s only a question of time.” He ignores America’s past success in stemming proliferation—including in South Korea. It is unprecedented for an American leader to accept proliferation as inevitable because America is “weak.” Secretary of State Henry Kissinger, someone whom Trump praised as one of the “biggest diplomats in the country,” didn’t. In fact, he applied pressure on South Korea in 1975 to keep the country from going nuclear.

This isn’t a left-vs.-right issue—among the strongest opponents of nuclear proliferation was President Ronald Reagan. More than 30 years before Obama went to Hiroshima to warn about nuclear war, it was Reagan who went to Tokyo to state definitively, “A nuclear war can never be won and must never be fought,” and to pledge that “our dream is to see the day when nuclear weapons will be banished from the face of the Earth.”

In the meantime, Reagan carefully pursued policies that sought to prevent the spread of those weapons. When he declared while seeking reelection in 1984 that he would “make America great again,” he spoke of the need to “reduce the risk of nuclear war by reducing the levels of nuclear arms.” He reflected on how he had spoken “to parliaments in Europe and Asia during these last three and a half years, declaring that a nuclear war cannot be won and must never be fought. And those words, in those assemblies, were greeted with spontaneous applause.” Building on Kissinger’s efforts, Reagan was able to get South Korea to give up its nuclear program—a legacy Trump risks throwing out—and a few years later did the same with Taiwan. He did so in part with subtle diplomacy, and by reassuring allies like those two countries that America stood with them. Trump, with his bombastic style, is not seen as reassuring among world leaders.