NATO collapse causes Russian and Chinese adventurism and cascading proliferation.

Thankfully, those predictions turned out to be wrong. There are multiple reasons for that, but one big one — one that also helped keep relations between other historical enemies, like South Korea and Japan, peaceful — is a shared participation in US alliance networks. The US serves as the ultimate security blanket, preventing these countries from having to build up their own armaments and thus risk a replay of World War I. But if American alliance commitments become and remain less credible, it’s possible this order could crack up. America’s partners aren’t stupid. They understand that Trump is the product of deep forces in American politics, and that his victory might not be a one-off. If they think that this won’t be the last “America First” president in modern history, depending on America the way that they have in the past could quickly become a nightmare. The worst-case scenarios for a collapse in the US alliance system are terrible. Imagine full Japanese and German rearmament, alongside rapid-fire proliferation of nuclear weapons. Imagine a crack-up of NATO, with European powers at loggerheads while Russia gobbles up the Baltic states and the rest of Ukraine. Imagine South Korea’s historical tensions with Japan reigniting, and a war between those two countries or any combination of them and China. All of this seems impossible to imagine now, almost absurd. And indeed, in the short run, it is. There is no risk — zero — of American allies turning on each other in the foreseeable future. And it’s possible that the next president after Trump could reassure American allies that nothing like this could ever happen again. But the truth is that there’s just no way to know. When a fundamental force for world peace starts to weaken, no one can really be sure how well the system will hold up.