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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 Imperial Diet 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.
In 1619 Frederick V of the Palatinate (successor to Frederick IV) accepted the crown of Bohemia, and in 1620 the Protestant Union signed the Treaty of Ulm, declining 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 its 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.
The Protestant Union developed in order to resist all hierarchic and un-Protestant tendencies within the different churches, and the preservation of the rights, honor, and liberty of German Protestantism. The Union also strove to maintain the Christian respect among the various denominations and their members. 
The Protestant Union became very active during the early 1600s following the election of Ferdinand II to the crown. The letter of Majesty had guaranteed the rights of Protestants, among them religious freedom. However, it was ambiguous on one point: whether lands of the Catholic Church were held from the King. If they were, then Protestants could build churches and worship freely on these crown lands; if not, then the Catholic landholder could prohibit Protestantism on his ecclesiastical property. But in 1617 the ecclesiastical rulers of Broumov and Hroby took over or destroyed Protestant churches and forbid further heretical worship. This angered the Protestant Union and so they defiantly assembled in Prague on May 23, 1618 and went into the chamber of four Catholic Lord Regents in what is known as the Second Defenestration of Prague. This was a key point in the Protestant Union because it was a time when they were defiant of the Emperor in order to preserve the Protestant rights they had received in the Peace of Augsburg and in the Letter to Majesty.
Guidelines of the Protestant Union
Wanting to strengthen the peace given by the Peace of Augsburg, Protestants formed a Union in order to advance their well-being, land, and people. Within this union, the Protestant leaders created guidelines and agreements to live by as follows:
- Each member shall keep in good faith with the order and their heirs, land and people, and no one shall enter into any other alliance.
- Each member of the union should keep a secret correspondence effectively to inform each other of all dangerous and offensive affairs which may threaten each other's heirs, land and people, and to this purpose each will keep in good contact with one another.
- Whenever important matters arise that concern the well-being of the union, the members of the union will help each other with faithful advice in order to uphold each and every one as much as possible.
- The wish of the union in matters concerning the liberties and high jurisdictions of the German Electors and Estates should be presented and pressed at subsequent Imperial and Imperial Circle assemblies, and not merely left to secret correspondence with each other.
- The union shall not affect our disagreement on several points of religion, but that notwithstanding these, we have agreed to support each other. No member is to allow an attack on any other in books or through the pulpit, nor give cause for any breach of the peace, while at the same time leaving untouched the theologian's rights of disputation to affirm the word of God.
- If one of the members of the union is attacked, the remaining members of the union shall immediately come to his aid with all the resources of the union. 
- 1555: The Peace of Augsburg was signed by Charles V and Lutheran Princes. This treaty allowed Roman Catholic and Lutheran Princes the right to decide which freedom their respective state would be under. However, it did not recognize Calvinist people and so many of the Protestant people were still in danger under this new treaty.
- 1608: Protestant Princes formed the alliance known as Protestant Union.
- 1609: The Catholic League was created after the creation of the Protestant Union
- 1609: Emperor Rudolph II passed the Letter of Majesty, which granted freedoms of religion to nobles.
- 1612: Matthias became Holy Roman Emperor. However, he was childless, so he passed his crown onto his cousin, Ferdinand of Styria.
- 1617: Ferdinand II was elected King of Bohemia and then later elected Holy Roman Emperor.
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- Hofmann, H.H. "The Crown & The Cross: The Protestant Union, 1608." The Crown & The Cross: The Protestant Union, 1608. N.p., n.d. Web. 25 Oct. 2012. <http://www.oocities.org/athens/sparta/5414/ProtestantUnion.htm>.
- Atkinson, Chris. "The Bohemian Rebellion." The Thirty Years' War: The Bohemian War. N.p., 2005. Web. 25 Oct. 2012. <http://www.pipeline.com/~cwa/Bohemian_Phase.htm>.
- Rickard, J. (17 November 2000), Thirty Years' War (1618–48), http://www.historyofwar.org/articles/wars_thirtyyears.html
- "Protestant Union (German)." Protestant Union (German). Christian Classics Ethereal Library, 3 June 2004. Web. 25 Oct. 2012. <http://www.ccel.org/s/schaff/encyc/encyc09/htm/iv.v.xlvi.htm>.
- Helfferich, Tryntje. The Thirty Years' War: A Documentary History. Indianapolis: Hackett Pub., 2009. Print.
- Bohemian Protestants and the Calvinist Churches.
- Odložilík, Otakar. Church History, Vol. 8, No. 4 (Dec., 1939), pp. 342–355. Published by: Cambridge University Press on behalf of the American Society of Church History. Article Stable URL: http://www.jstor.org/stable/3160169