quality:correction
Background: 1556 to 1618

Disputes occasionally resulted in full-scale conflict like the 1583 to 1588 Cologne War, caused when its ruler converted to Calvinism. More common were events such as the 1606 "Battle of the Flags" in Donauwörth, when riots broke out after the Lutheran majority blocked a Catholic religious procession. Emperor Rudolf approved intervention by the Catholic Maximilian of Bavaria. In return, he was allowed to annex the town, and as agreed at Augsburg, the official religion changed from Lutheran to Catholic.

When the Imperial Diet opened in February 1608, both Lutherans and Calvinists sought formal re-confirmation of the Augsburg settlement. In return, the Habsburg heir Archduke Ferdinand required the immediate restoration of all property taken from the Catholic Church since 1555, rather than the previous practice whereby the court ruled case by case. This demand threatened all Protestants, paralysed the diet, and removed the perception of Imperial neutrality.

Loss of faith in central authority meant towns and rulers began strengthening their fortifications and armies; outside travellers often commented on the growing militarisation of Germany in this period. In 1608, Frederick IV, Elector Palatine formed the Protestant Union, and Maximilian responded by setting up the Catholic League in July 1609. Both were created to support the dynastic ambitions of their leaders, but combined with the 1609 to 1614 War of the Jülich Succession, the result was to increase tensions throughout the empire. Some historians who see the war as primarily a European conflict argue Jülich marks its beginning, with Spain and Austria backing the Catholic candidate, France and the Dutch Republic the Protestant.

External powers became involved in what was an internal German dispute due to the imminent expiry of the 1609 Twelve Years' Truce, which suspended the Eighty Years' War between Spain and the Dutch Republic. Before restarting hostilities, Ambrosio Spinola, commander in the Spanish Netherlands, needed to secure the Spanish Road, an overland route connecting Habsburg possessions in Italy to Flanders. This allowed him to move troops and supplies by road, rather than sea where the Dutch navy was dominant; by 1618, the only part not controlled by Spain ran through the Electoral Palatinate.

Since Emperor Matthias had no surviving children, in July 1617 Philip III of Spain agreed to support Ferdinand's election as king of Bohemia and Hungary. In return, Ferdinand made concessions to Spain in Northern Italy and Alsace, and agreed to support their offensive against the Dutch. Doing so required his election as emperor, which was not guaranteed; Maximilian of Bavaria, who opposed the increase of Spanish influence in an area he considered his own, tried to create a coalition with Saxony and the Palatinate to support his candidacy.

Another option was Frederick V, Elector Palatine, a Calvinist who succeeded his father in 1610, and in 1613 married Elizabeth Stuart, daughter of James I of England. Four of the electors were Catholic, and three were Protestant; if this balance changed, it would potentially result in the election of a Protestant emperor. When Ferdinand became king of Bohemia in 1617, he also gained control of its electoral vote; however, his conservative Catholicism made him unpopular with the predominantly Protestant nobility, who were also concerned about the erosion of their rights. These factors combined to bring about the Bohemian Revolt in May 1618.