Disputes occasionally resulted in full-scale conflict like the 1583 to 1588 Cologne War, caused when its ruler converted to Calvinism. More common were events such as the 1606 'Battle of the Flags' in Donauwörth, when riots broke out after the Lutheran majority blocked a Catholic religious procession. Emperor Rudolf approved intervention by the Catholic Maximilian of Bavaria, who was allowed to annex the town, changing it from Lutheran to Catholic under the principle of "cuius regio, eius religio" ("whose realm, their religion").