Political Trenches: War, Partisanship, and Polarization

We show how local segregation and exposure to partisans affect political behavior and polarization, contributing to critical ideological realignment. We exploit large-scale, exogenous, and high-stakes peer assignment due to the universal conscription of soldiers from each of 34,947 French municipalities into infantry regiments during WWI. Soldiers from poor, rural municipalities---where the novel redistributive message of the left had yet to penetrate---vote more for the left after the war when exposed to left-wing partisans within their regiment, while neighboring municipalities assigned to right-wing partisans become inoculated against the left. We provide evidence that these differences reflect persuasive information by trusted peers and officers, combined with material incentives, rather than pure conformity. They further induce sharp and enduring post-war discontinuities across regimental catchment boundaries, reflected not only in divergent voting patterns but also in violent civil conflict between Collaborators and the Resistance during WWII.