We exploit the fact that disagreements in the German leadership after the Franco-Prussian War in 1870 led to a quasi-exogenous division of Alsace and Lorraine to provide rare evidence on group identity formation within historically homogeneous regions. In line with the rejection-identification hypothesis, people in the treated area which experienced a change in nation-status and were exposed to repressive homogenization policies express a stronger regional identity and support more regional autonomy today. On average, subjects with a stronger regional also express a stronger European identity, which we exploit in a regression discontinuity design at the municipal level to reveal whether these identity differences are causal. We find that support for the European Union is significantly stronger in two crucial referenda, a result that is robust across different specifications and bandwidths, and not driven by language differences, large agglomerations or distance to foreign countries. The effect seems to be the strongest for the first two age cohorts after WW2 and diminishes for later generations.