Abstract: This paper provides the first empirical evidence for the effect of inclusive propaganda on cultural assimilation. During World War I, the U.S. witnessed unprecedented propaganda led by the government to garner public support for the war. Given that 15% of the American population was foreign-born in 1917, the government promoted national unity and the integration of immigrants into the country's shared culture and values. I create a novel dataset of city-level wartime campaigns by the Committee of Public Information (CPI), the first federal bureau charged with large-scale propaganda in American history. Leveraging idiosyncratic cancellation in campaign events with a difference-in-differences strategy, I find that immigrants living in cities with higher exposure to the CPI's inclusive propaganda were more likely to initiate naturalization, marry native-born spouses, and give their children American names. Moreover, these immigrants supported the country even more actively than natives during the war by purchasing more war bonds and saving more food. Further heterogeneity analysis shows that immigrants who were more culturally distant from natives -- either due to war-induced distance like wartime allegiances or existing cultural gaps -- exhibited greater assimilation efforts, corroborating existing theoretical models. My findings reveal that inclusive messages from governments have the power to unify their populations.