This case study of the Alsatian community of Sainte-Marie-aux-Mines argues that although negotiating religious differences at the local level was essential to defining community, the political context was essential in setting the framework. Straddling the Lorraine/Alsace border, Sainte-Marie was divided among Catholics, Lutherans, Calvinists, Zwinglians and Anabaptists as well as between French- and German-speakers. After the Thirty Years' War, the changing demographic balance among the confessional groups, especially the immigration of Catholics necessitated a re-evaluation of Catholic/Protestant coexistence. The Peace of Westphalia established a legal framework in the imperial territories based on cuius regio eius religio. France's 1648 annexation of most of Alsace, meant that French centralising authority collided with this seigneurial territorial system. The French crown could not, however, govern without the co-operation of the local authorities: religious groups at Sainte-Marie exploited the resulting ambiguities. In uneasy coexistence Catholics enjoyed royal favour and Protestants had the protection of the local seigneurs. This local outcome mirrored the imperfect process by which the French monarchy imposed itself on the imperial system of Alsace.