When do groups on the social periphery assimilate into the social core of a nation? Building on a diverse set of literatures, I argue that individual participation in military service creates a number of conditions that drive individuals to assimilate into a broader national culture. To test the theory, I focus on the case of World War I in the United States–a period that closely followed a massive wave of immigration into the United States. Using an instrumental variables strategy leveraging the exogenous timing of the war, I show that individuals of foreign, European nativity–especially, the Italians and Eastern Europeans–were more likely to assimilate into American society after serving in the U.S. military. The theory and results contribute to our understanding of the ways in which states make identity and the prospects for immigrant assimilation in an age without mass warfare.