The Protestant Union or League of Evangelical Union (also known as the Evangelical Union or Union of Auhausen) was a coalition of ProtestantGerman states that was formed in 1608 to defend the rights, lands and person of each member.
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 May1621, formally dissolved the Protestant Union.