In the past several months, a meaningful debate has finally started to emerge over America’s role in the world. Politicians and analysts — left, right and center — are conceding that longstanding mistakes have brought the United States to an uncertain moment. Provoked by President Trump, they are concluding that the bipartisan consensus forged in the 1990s — in which the United States towered over the world and, at low cost, sought to remake it in America’s image — has failed and cannot be revived.