We study the dynamics between local segregation, partisanship, and political polarization. We exploit large-scale, exogenous and high-stakes peer assignment due to universal conscription of soldiers assigned from each of 34,947 municipalities to French infantry regiments during WWI. We find that municipalities with soldiers serving with the same line regiment converge in their post-war voting behaviors. Soldiers from rural municipalities exposed to more leftist regimental peers become more leftist for the first time after the war, while adjacent municipalities assigned to the right are inoculated against the left. We provide evidence that these differences reflect persuasive information exchanged among peers when the stakes for cooperation and trust are high rather than group conformity. These differences further lead to the emergence of sharp and enduring post-war discontinuities across 435 regimental boundaries that are reflected, not only in voting, but also in violent civil conflicts between Collaborators and Resistants during WWII.
