In 1914, the assassination of Archduke Franz Ferdinand—who was in line for the throne of the Austro-Hungarian Empire—propelled Europe into a war unlike any the world had ever seen. But it would take yet another remarkable series of events for the United States to decide to enter the fray, a century ago in April of 1917. By the time the armistice arrived the following year, empires had fallen and 15 million combatants lay dead. And for the United States, the consequences of the decision to get involved would reverberate throughout would come to be known as the American Century, and still echo today.