Global power structures that have guided us and kept the peace since World War II are under never-before-seen stress. Populations and economies are concentrating in urban centers, mass migrations continue to reshape national identities, climate change is reshaping landscapes, and democracies seem imperiled in places they once seemed indomitable. Tech companies today now rival entire nations in terms of reach and user populations, raising new questions about privacy and security. Tools that used to be exclusive to nation-states—from drones to hacking to space rockets—have been democratized, spreading to entrepreneurs as well as terrorists and criminals. Social media — often populated by unverified stories and dubious sources — has superseded traditional media empires that once held sway over public discourse. In what many are calling the post-American world order, who will lead?