Six months after Woodrow Wilson thrust the United States into Europe's World War, Vladimir Lenin overthrew Russia's democratic revolution, setting up the repressive regime of the Soviet Union. Traditionally, countries fought to increase or protect their national interests, but after 1917, they began going to war over ideas. As Arthur Herman argues here, Lenin and Wilson both unleashed disruptive ideologies—nationalism and globalism versus communism and terrorism—that have shaped the world for the past century, and continue to do so today.