The recent ascent of right-wing populist movements in several countries has rekindled interest in understanding the causes of the rise of fascism in the interwar years. In this article, we argue that there was a strong link between the surge of support for the Socialist Party after World War I and the subsequent emergence of fascism in Italy. We first develop a source of variation in socialist support across Italian municipalities in the 1919 election based on war casualties from the area. We show that these casualties are unrelated to a battery of political, economic, and social variables before the war and had a major effect on socialist support (partly because the socialists were the main antiwar political movement). Our main result is that this boost to socialist support (that is “exogenous” to the prior political leaning of the municipality) led to greater local fascist activity as measured by local party branches and fascist political violence, and to significantly larger vote share of the Fascist Party in the 1921 and 1924 elections. We provide evidence that landowner associations and greater presence of local elites played an important role in the rise of fascism. Finally, we find greater likelihood of Jewish deportations in 1943–45 and lower vote share for Christian Democrats after World War II in areas with greater early fascist activity