A coalition of Protestant German states from the Protestant Union or League of Evangelical Union (also known as the Evangelical Union or Union of Auhausen), to defend the rights, lands and person of each member following the establishment, by the Holy Roman Emperor and Duke Maximilian I of Bavaria, of Roman Catholicism in Donauwörth in 1607 and after a majority of the Reichstag have decided in 1608 that the renewal of the Peace of Augsburg of 1555 should be conditional upon the restoration of all church land appropriated since 1552.

Meeting on May 14, 1608, in Auhausen, near Nördlingen, the Protestant princes of the Palatinate, Anhalt, Neuburg, Württemberg, Baden, Ansbach, Bayreuth, Hesse-Kassel (or Hesse-Cassel), Brandenburg, Ulm, Strasbourg and Nürnberg form a military league under the leadership of Frederick IV of the Palatinate.

The Protestant Union is weakened from the start by the non-participation of several powerful Protestant rulers, such as the Elector of Saxony.

The Union is also beset by internal strife between its Lutheran and Calvinist members.