The mushroom cloud of the first test of a hydrogen bomb. (Photo by DSWA-DASIAC, REUTERS)

According to Donald Trump, the United States should not try so hard to stop nuclear proliferation. On Sunday night, during a Republican town hall hosted by CNN’s Anderson Cooper, Trump declared that proliferation is “going to happen anyway.” And just a week earlier, Trump told the New York Times, “If Japan had that nuclear threat, I’m not sure that would be a bad thing for us.” Nor would it be so bad, he’s said, if South Korea and Saudi Arabia had nuclear weapons, too.

We can break down Trump’s assertions into two ideas: Proliferation is inevitable, and it is good for the United States — at least when its allies are the ones going nuclear. What can political science tell us about each of these beliefs?

It turns out that both propositions fly in the face of a wide range of recent scholarship.

Is nuclear proliferation inevitable?

Trump’s logic for this idea is based on his belief that the United States is weak and that past proliferation ensures future proliferation. Here’s what Trump told the Times about Japan: “If the United States keeps on … its current path of weakness, [Japan is] going to want to have [nuclear weapons] anyway with or without me discussing it.”

Trump also implied that South Korea and Japan would inexorably seek nuclear weapons — regardless of what the United States does — because so many countries have already gone nuclear. As he said to Anderson Cooper: “It’s only a question of time. … You have so many [nuclear] countries already.”

But as we show in a number of research articles, those assumptions don’t match the historical record. For the past 70 years, through mutually reinforcing policies — including security guarantees, troop deployments, arms sales, nuclear umbrellas and sanctions threats — U.S. administrations from both parties have inhibited nuclear proliferation.

When another country built nuclear weapons, the United States limited the repercussions by discouraging that country from conducting nuclear tests.

What about Trump’s belief that U.S. allies will inevitably seek nuclear weapons because the United States is economically and militarily weak? That doesn’t match the facts, either. The United States remains the world’s dominant military power — it spends three to four times as much on its military than China does, and it has the world’s most advanced nuclear arsenal. The United States also has a dynamic and growing economy, while its rivals’ economies are slowing or even declining.

But even when the U.S. economy was flagging, the government successfully prevented other countries from acquiring nuclear arms. The 1970s were a period of high inflation and low economic growth in the United States. Yet that’s when Washington launched some of its most determined and successful nonproliferation efforts, including founding the Nuclear Suppliers’ Group, a global body that restricts the spread of sensitive nuclear technology, and passing laws that imposed mandatory nonproliferation sanctions, which have successfully deterred other countries from embarking on nuclear weapons programs.

Trump’s foreign policies would make his predictions come true

Although history suggests that proliferation is not inevitable, recent research on nonproliferation suggests that Trump’s proposed foreign policy might make it so.

Trump says he would scale back or entirely end U.S. alliance commitments unless our allies made major financial concessions. In his interview with the Times, Trump said that the United States “take[s] tremendous monetary hits on protecting countries” such as Japan, South Korea, Germany and Saudi Arabia. He also denounced the U.S.-Japan Mutual Security Treaty as “one-sided,” said that the United States doesn’t need to maintain forces in South Korea and described the North Atlantic Treaty Organization as “obsolete.”

But if those security institutions and military deployments disappeared, U.S. allies — including Japan and South Korea — might well pursue nuclear weapons of their own. Recent research shows that alliances are a powerful tool for preventing proliferation, both because they reassure states that their security will be protected in case of attack and because they give senior partners the leverage to restrain their allies’ nuclear ambitions. Research also demonstrates that the type of U.S. troop withdrawals Trump envisions have a history of prompting allies to consider developing their own nuclear weapons.

Consider the last time the United States had a president who was skeptical about nonproliferation and who tried to reduce U.S. commitments to its allies in Asia. As part of his Guam Doctrine — a plan to increase Asian allies’ military self-reliance — President Nixon withdrew 20,000 troops from South Korea. Famously, he also traveled to China to improve Sino-American relations. As a result, South Korea launched a covert nuclear weapons program, and Taiwan ramped up its own nuclear ambitions. So why didn’t they end up with nuclear weapons? The administrations that followed Nixon’s redoubled efforts to stop them.

Research does not support the idea that the spread of nuclear weapons is inevitable. But isolationist “America First” policies could prompt that spread. Defining U.S. strategic interests primarily in terms of monetary gain, and curtailing U.S. global engagement toward that end, would boost the probability that our allies would respond by going nuclear.

Would nuclear proliferation be good for U.S. interests?

What about Trump’s second proposition: that proliferation by our allies would be good for U.S. interests? This argument is based on the idea that nuclear-armed allies could help contain U.S. adversaries and enable the United States to save money. As Trump told Cooper, “I would rather see Japan having some form of defense, and maybe even offense, against North Korea.” And as he suggested, the United States can’t afford to protect Japan and South Korea — and therefore, “they have to pay us or we have to let them protect themselves.”

Reducing military commitments and letting allies build their own nuclear weapons might save money for the United States. But international relations scholarship suggests that allied proliferation would have broader negative repercussions. Among these would be declining U.S. influence. When nations gain their own military capabilities, they rely less on their allies and become less subject to their sway. And that can undermine a senior partner’s ability to hold its junior allies back from risky military actions.

In other words, allowing or encouraging proliferation would worsen the “American weakness” that Trump decries.

Recent nonproliferation research underscores this proposition. Mark Bell shows that nuclear allies are likely to become more independent of their patrons and in some cases can develop more assertive foreign policies. And Francis Gavin and Matthew Kroenig show that the fear of declining influence was one reason why most American administrations vigorously opposed the spread of nuclear weapons.

Nuclear allies can also become security risks. Vipin Narang demonstrates that when weaker states gain nuclear weapons, they often seek to coerce their senior partners into intervening on their behalf by threatening to use nuclear weapons. That’s what Israel did at the height of the 1973 Arab-Israeli War. That’s what South Africa did during its 1988 confrontation with Cuban forces in Angola. And that’s what Pakistan did in the midst of its 1990 military crisis with India.

Instead of relieving the United States of a military burden, as Donald Trump suggests, having more nuclear allies could increase the risk that the United States would get involved in conflicts that might turn nuclear.

Furthermore, were South Korea or Japan to begin developing nuclear weapons, their rivals might be tempted to launch preventive military strikes, which research suggests has been frequently considered in the past. The road to nuclear acquisition is often rocky and increases the likelihood of militarized conflict. For example, Soviet worries that West Germany would acquire nuclear weapons helped trigger the Berlin Crisis.

And if Japan or South Korea actually acquired nuclear weapons, we could possibly see a nuclear arms race in Asia. Japan’s neighbors, including South Korea, would fear resurgent Japanese militarism. North Korea would expand its nuclear capabilities. China would continue to expand its own nuclear arsenal.

Why haven’t we seen nuclear arms races before?

Nuclear “domino effects” have not been common historically. But that’s largely because of determined U.S. efforts to stop them.

Since the dawn of the nuclear age, the United States has pursued nonproliferation as a top policy priority. That includes sponsoring and enforcing the Nonproliferation Treaty (NPT). Research suggests the NPT has been instrumental in limiting the spread of nuclear weapons, in part by coordinating states’ beliefs about one another’s nonproliferation commitments. To develop nuclear weapons, Japan and South Korea would need to violate or withdraw from the NPT. That could prompt U.S. allies and adversaries in other regions — including Saudi Arabia, Germany and Iran — to question the treaty’s viability and consider seeking their own nuclear arsenals.

Would this be so bad? After all, no two nuclear armed states have fought a major war with each other, and nuclear weapons have not been used in conflict since the United States bombed Hiroshima and Nagasaki in 1945.

But the conclusion that nuclear weapons produce peace is subject to debate. It’s true that there has been no war between major powers since 1945. But that may be due to other factors. The quantitative evidence linking nuclear weapons to a reduced risk of conflict is limited at best.

Further, theoretical and historical evidence suggests that nuclear accidents and miscalculations are likely. More countries with nuclear weapons would mean more opportunities for catastrophic nuclear mistakes.

So what’s the takeaway?

A look at history shows us that nuclear proliferation is anything but inevitable. U.S. nonproliferation efforts have been surprisingly successful, even when the United States was weaker than it is today.

Without firm U.S. opposition to the spread of nuclear weapons — a policy implemented through “carrots” like alliances and “sticks” like sanctions — the world would probably have far more than nine countries with nuclear weapons. What’s more, research suggests that nuclear proliferation would reduce U.S. world influence, undermine global stability and increase the risk of nuclear war.

Gene Gerzhoy is a congressional fellow with the American Political Science Association.

Nick Miller is an assistant professor of political science and international and public affairs at Brown University.