What factors help to explain the cultural integration of immigrants? I argue that individual participation in military service creates a number of conditions that drive individuals to integrate into a broader national culture. To test the hypothesis, 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 data on the entire population of immigrants in the U.S., I show that men of foreign, European nativity were more likely to integrate into American society after serving in the U.S. military. Heterogeneous effects suggest some scope for the importance of wartime social networks. This article contributes to our understanding of the ways in which states make identity and the prospects for immigrant integration in an age without mass warfare..