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The 'Protestant Union' (also known as 'Evangelical Union' or 'Union of Auhausen') was a coalition of Protestant German states that was formed in 1608 to defend the rights, lands and person of each member.
It was formed after the Holy Roman Emperor tried in 1607 to reestablish Catholicism in Donauwörth and after a majority of the Reichstag decided that the renewal of the Peace of Augsburg of 1555 should be conditional upon the restoration of all church land appropriated since 1552. The Protestant Princes met in Auhausen, near Nördlingen and on May 14 1608 formed a military league under the leadership of Frederick IV of the Palatinate. In response, the Catholic League was formed in the following year, headed by Duke Maximilian I of Bavaria.
Members included the Palatinate, Anhalt, Neuburg, Württemberg, Baden, Ansbach, Bayreuth, Hesse-Kassel, Brandenburg, Ulm, Strasbourg and Nürnberg.
The Protestant Union was weakened from the start by the non-participation of powerful Protestant rulers such as the Elector of Saxony. The Union was also beset by internal strife between its Lutheran and Calvinist members.
When Frederick V of the Palatinate, (successor to Frederick IV), accepted the crown of Bohemia in 1619 the Protestant Union signed the Treaty of Ulm (1620) and refused to support him. In January 1621 the Holy Roman Emperor Ferdinand II imposed the Ban of the Empire upon Frederick V and gave the Lower Palatinate to Maximilian. The Protestant Union met in Heilbronn in February and formally protested over the actions of Ferdinand. Ferdinand ignored this complaint and ordered the Protestant Union to disband their army. In May under the Mainz Accord the members of the Protestant Union complied with Ferdinand's demand and on 24 May 1621 formally dissolved the Protestant Union.