The post-World War I era witnessed a dramatic resurgence of the Ku Klux Klan, with membership swelling to millions across the United States during the 1920s. Drawing on newly digitized military records and KKK membership rolls, we examine the relationship between military service and subsequent Klan participation among white veterans. Reconstructing the WWI draft lottery, we find that white men who were randomly inducted into the Army were significantly more likely to join the KKK than comparable non-drafted men from the same communities. This effect was particularly pronounced among those conscripted from regions with stronger expressions of patriotic sentiment. Our analysis reveals that increased KKK membership was significantly mediated by contact with Black and Jewish soldiers, with the strongest effects observed among white veterans with the most exposure to those groups. These findings contribute to our understanding of how wartime experiences shaped racial attitudes and mobilized white supremacist activity in interwar America.