World War I is the most important single event in the history of globalization. The war ended the first significant era of increasing economic ties among nations and thereby shaped the economic history of the twentieth century. The war set off both a search for ways to re-create the prewar liberal world economy and attempts to create statist alternatives to it. The collapse of interbank cooperation and expansion of controls on trade, migration, and agriculture meant that economic globalization re-emerged only very slowly over the rest of the twentieth century. Indeed, the long-term effects of World War I lasted until the 1990s. The lesson of this story for the twenty-first century is to check the dangers inherent in a multipolar world, where globalization produces both economic growth and social tensions.