President Trump’s remarks in recent weeks — contending that fellow NATO members “owe [the United States] a tremendous amount of money,” labeling the European Union a trade “foe” and calling Russian President Vladimir Putin “a good competitor,” for example — have heightened the anxiety of observers who question the resilience of the postwar order. Some focus on the challenges posed by external actors — whether the selective revisionism of China as a complex competitor-cum-partner or the more confrontational behavior of Russia, which appears to have calculated that it can accrue more short-term influence by destabilizing the system than by integrating into it.