One of the EU’s main architects, Jean Monnet, a French diplomat and economist, spent much of World War II in Washington, DC, as a negotiator for the European allies. After Germany’s defeat, he was convinced that only a united Europe could prevent another devastating war in the West. “There will be no peace in Europe,” he wrote in his memoir, “if states are reconstituted on the basis of national sovereignty.”Almost everyone on the European continent, exhausted by war, and faced with the shattered institutions of their ravaged nation-states, agreed. Only the victorious British, with their old institutions more or less...