Global mass migration was one of the most defining features of the Gilded Age and the Progressive Era. But so was intense xenophobia. This article offers a new definition of xenophobia and examines how xenophobia helped to drive some of the most defining features of the Gilded Age and the Progressive Era, including progressive reform, white supremacy, the expanded capacity and power of the nation-state, and the growth of U.S. global power and influence. It draws a connection to contemporary America, where, under the Trump administration, xenophobia is transforming a wide range of public policies, legitimizing racism and white supremacy, and impacting U.S. foreign relations.