How did the Protestant Union form in 1608?

The Protestant Union was formed in 1608 as a defensive military alliance among German Protestant states to protect against the growing influence of the Roman Catholic Counter-Reformation.1

The formation was triggered by rising religious tensions within the Holy Roman Empire. In February 1608, Catholic princes at the Diet (Reichstag) moved to reclaim secularized church lands. When this motion was rejected, a group of Protestant princes protested and withdrew from the assembly.1

Following this, representatives from several states—including the elector Palatine of the Rhine, the dukes of Neuburg and Württemberg, and the margraves of Baden-Durlach, Ansbach, and Kulmbach—met at the monastery of Auhausen. On May 14, 1608, they established a 10-year defensive pact. The alliance was led by Frederick IV, Elector Palatine, who had long sought to unite German Protestants against the Catholic Church and the Habsburgs. The union was further galvanized by the forced occupation of the city of Donauwörth by Catholic forces in 1607, which convinced many Protestants that a broader Catholic offensive was underway.123