We use a policy experiment in Indonesia to show how local political boundaries affect ethnic tension. Redrawing district borders along group lines reduces conflict. However, the gains in stability are undone or even reversed when new boundaries increase ethnic polarization. Greater polarization leads to more violence around majoritarian elections but has little effect around lower-stakes, proportional representation elections. These results point to distinct incentives for violence in winner-take-all settings with contestable public resources. Overall, our findings illustrate the promise and pitfalls of redrawing borders in diverse countries where it is infeasible for each group to have its own administrative unit.