"Pure Americanism": World War I and the Rise of the Ku Klux Klan

The post-World War I era witnessed a dramatic resurgence of the Ku Klux Klan, with membership swelling to millions across the United States. Assembling the largest known collection of Klan rosters, we show that the organization drew disproportionately from civic and professional elites -- public administrators, law enforcement officers, and medical professionals -- with veterans significantly overrepresented in its ranks. Digitizing tens of millions of military records and reconstructing the national draft lottery, we demonstrate the causal impact of military service in fueling the Klan's rise. White men randomly inducted into the military were more than twice as likely to join the KKK as comparable undrafted men from the same area. These effects were accompanied by other nativist shifts -- as revealed by spousal choice, child naming, and party affiliation -- but not by increased fraternal participation or economic status. Examining variation in soldiers' training camps, units, and roles, we find suggestive evidence of the mediating role of nationalistic indoctrination, racial threat, and peer exposure. Leveraging detailed pre-war address records, we document significant spillovers at multiple geographic levels, both within veteran households and across their neighborhoods. Our findings illuminate how military service shaped racial attitudes and fueled the spread of white supremacy in early 20th-century America.