There is no shortage of scenarios in which America’s postwar world comes under challenge and starts to crack. It could take the form of a draining showdown with Islamist radicals in the Middle East, a conflict with Russia that engulfs Europe, or a one-on-one superpower naval battle with China. Soon after his election, Donald Trump finished his first conversation as president-elect with Barack Obama at the White House fretting about the threat from a nuclear-armed North Korea.
