Of all the outrageous things Donald Trump has said, his proposal to withdraw US military support from Japan and South Korea, and even encourage them to acquire nuclear weapons, might not sound particularly egregious.
But this would be a big deal, overturning 70 years of American foreign policy with potentially sweeping implications.
"This is basically like, 'Hey, maybe we should think about communism,'" Jennifer Lind, a professor at Dartmouth who studies East Asia, tells me. "With one blasé comment, this entire foundation of US grand strategy is just blasted away."
Here's what this policy could mean if implemented by an actual Trump administration — and why Asia watchers find it so disconcerting.
Why Japan and South Korea don't have nukes
When Trump is talking about ending US security support for Japan and South Korea, he is putting both of those countries in situations where it becomes more likely that their leaders would desire and maybe even seek nuclear weapons.
East Asian security is a very delicate balance between several competing military powers, but it's stable due in large part to political choices made by the regional powers and the United States for the past 70 years.
After Japan's defeat in World War II, the country's new constitution declared that the country would be officially pacifist, meaning its military would be limited to self-defense only. While the constitution was imposed by the US occupation forces, this was also an earnest expression of Japan's postwar regret.
This also played out in Japan's rejection of nuclear weapons, which was rooted as well in its experience of having suffered two nuclear bombings during the war.
"Japan, as a country, has deep-seated anti-nuclear sentiment, based on the variety of experiences they've had with this technology," Lind explains.
Over time, Japan's nuclear taboo might have waned, scholars say, if not for its mutual defense treaty with the United States. As part of that treaty, the US pledged its own nuclear weapons to Japan's defense, meaning Japan had little reason to consider developing its own weapons — even as the nearby Soviet Union and China and, much later, North Korea joined the nuclear club.
"At various times, Japanese leaders have seriously considered developing a nuclear arsenal," Nick Miller, a professor at Brown University who studies nuclear proliferation, explains. "Every time, they concluded it's not in their interests. ... The US security guarantee/US nuclear umbrella provides for their security without them having nuclear weapons."
This goes to show the importance of America's role in protecting Japan from having to defend itself, which might lead it to desire nuclear weapons. Though the vast majority of Japanese oppose developing nuclear weapons — 80 percent, according to a 2006 poll — it's at least possible these numbers could shift absent US military support.
South Korea, like Japan, is surrounded by nuclear powers, and like Japan it has not developed its own nukes, in part because it is under US protection. But the nuclear taboo is not quite as strong there — one poll found 68 percent support developing nuclear weapons, though another found only 53 percent support — and in the 1970s the country had a short-lived covert nuclear program.
That program ended in part because the Nixon administration pressured the country to halt the program — showing the importance of the United States' role in preventing nuclear proliferation there.
"The US found out, and cracked down pretty hard on South Korea — basically threatened to withdraw the security commitment and cut off economic aid," Miller explains. "South Korea was basically coerced into not developing these capacities."
This is the flip side of US security assistance to these two countries. Because Japan and South Korea rely on US security support, the United States has a great deal of leverage over these two countries. Because the US opposes these countries building their own nukes, they're much less likely to jeopardize the alliance by pursuing them.
How Trump's policy could change things
Donald Trump's proposed South Korea and Japan policy would fundamentally alter the security situation in northeast Asia in ways that experts warn could lead Japan and/or South Korea to develop nuclear weapons.
The main goal of Trump's East Asia policy, according to Trump, is to limit the costs to America of protecting its allies. He wants to make the Japanese and South Koreans pay the US a cash tribute in exchange for the security guarantee; if they don't pay, Trump would withdraw American troops and protection.
"I would not do so happily, but I would be willing to do it," Trump said of pulling out to the New York Times. "We cannot afford to be losing vast amounts of billions of dollars on all of this."
This policy would have the effect of dramatically weakening the US security guarantee. Even if South Korea and Japan decided to pay up, they would know that the previously ironclad guarantee of American support was now more conditional. They would have to start thinking about taking steps to protect themselves from China and North Korea.
Steps like, for example, acquiring nuclear weapons.
Normally, the US would put all available pressure on these countries to make sure that didn't happen. But Trump has suggested otherwise — implying that he'd support Japan and South Korea developing nukes as a way to lower costs to the United States. As he put it in an interview with Anderson Cooper on Tuesday evening:
COOPER: It has been a U.S. policy for decades to prevent Japan from getting a nuclear weapon.
TRUMP: That might be policy, but maybe...
COOPER: South Korea as well.
TRUMP: Can I be honest are you? Maybe it's going to have to be time to change, because so many people, you have Pakistan has it, you have China has it. You have so many other countries are now having it.
It's this particular combination of policies — weakening the US security guarantee as well as the commitment to nonproliferation in East Asia — that makes a nuclear South Korea and/or Japan more likely in a world of a Trump presidency.
"If you're thinking about a scenario that could actually lead to Japan or South Korea getting nuclear weapons, it pretty much would be a US president who is not committed to nonproliferation and a president who wants to weaken the security guarantee," Miller says. "Trump has said things about both of those."
That risk could be especially high in South Korea.
"The Japanese, it's not remotely a foregone conclusion," Lind says. "South Korea is very different. I think it basically would be [automatic]."
Best case, nothing happens. Worst case?
"If you look at the way US policy has evolved since 1945, it's hard to find a policy in which US foreign policy has been more consistent than nonproliferation," Mark Bell, a research fellow at Harvard's Belfer Center for Science and International Affairs, says. "The US, since 1945, has been extraordinarily consistent in trying to prevent other countries from acquiring nuclear weapons."
As Miller points out in a 2014 paper, the conventional wisdom in the early to mid–Cold War was that nuclear weapons were very likely to spread around the world. That was prevented, in part, by a concerted US policy effort.
"Through a mix of bilateral diplomacy, threats of sanctions, security guarantees, and the promotion of the NPT [Non-Proliferation Treaty]," Miller writes, "the United States succeeded in slowing or rolling back several nuclear programs."
Trump would be tearing up this longstanding policy, taking a step into the unknown. Accurately predicting the consequences is thus essentially impossible.
Were South Korea or even Japan to acquire nuclear weapons, it's possible this would work out okay, deterring Chinese and North Korean aggression without substantially altering the region's security balance. Both Pakistan and India acquired nuclear weapons, and, so far at least, it hasn't led to World War III.
That's the best case. But there are plenty of plausible worse scenarios as well.
For example, if either country does decide to build nuclear weapons, it will take that country some time to develop its program, and to build enough of an arsenal to serve as a reliable deterrent. During this time, adversaries such as China or even North Korea would have an incentive to try to disrupt that development to maintain their nuclear superiority.
"You have a Trump presidency ... and he decides to pull out troops from Japan and South Korea, you have Japan and South Korea potentially racing to develop nuclear weapons without the benefit of US troops being there," Miller says. "That provides a lot of incentive for countries in the region like China or North Korea to try to stop that process."
As Bell puts it, ominously, "We're talking about the remote possibility of an actual nuclear war between Japan and China."
That possibility, it is worth stressing, is indeed extremely remote. The risk is not that, for example, China would simply launch a nuclear war against Japan, which would be far too dangerous and costly to be worth it. Rather, the risk is that, for example, China might try to bully or threaten Japan out of developing nuclear weapons, and that in a period of tension, this bullying could potentially spiral out of control into a full-blown conflict neither side actually wanted.
And there are other risks.
According to scholars, successful nuclear deterrence results in something called the stability/instability paradox: The fact that major wars are unlikely makes countries feel safer in engaging in small provocations against one another, knowing that nuclear deterrents make those small provocations unlikely to escalate to full-blown war.
Consider, for instance, the South and East China Seas — areas where Japan, South Korea, and China have territorial disputes. If the former two powers are nuclear-armed, and unrestrained by the United States, the chances of low-level conflict could go up.
"Certainly, we would be worried about these sort of lower-level, stability-instability paradox type things," Bell says.
That's not an exhaustive list of things that could happen if Trump were elected and followed through on these policies. Since no one can really know what will happen, there's no sense in listing every single hypothetical possibility.
These examples, rather, illustrate just how serious the ideas we're discussing are. It is very easy to detach ourselves from the potential consequences of a Trump presidency: to see his candidacy as clownish, and simply assume that his outlandish policy ideas would never be implemented. But Trump is the leading Republican candidate; it is time to take his ideas seriously.
And nothing is more serious than nuclear weapons.
So when Trump just throws out ideas like, "Let Japan and South Korea have nuclear weapons," with little apparent thought about the potentially catastrophic consequences, it should worry you.
"Things that are of that magnitude, you don't just word-vomit out into the dialogue," Lind says. "There's no sense whatsoever that he understands what our grand strategy is."