The Protestant Union or League of Evangelical Union (also known as the 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 and Duke Maximilian I of Bavaria reestablished Roman Catholicism in Donauwörth in 1607 and after a majority of the Reichstag had 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. 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 of Bavaria.
The Protestant Union was weakened from the start by the non-participation of several 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 his electorate and the Upper Palatinate to Maximilian. The Protestant Union met in Heilbronn in February and formally protested 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.