The Thirty Years' War, 1618-48

The anti-Habsburg rebellions reflected the rising tensions between Catholics and Protestants in the early 1600s. Proponents of the Counter-Reformation, often operating under Habsburg protection, were reaping the fruits of a generation of work: monastic life was reviving, Catholic intellectual life was regaining confidence, and prominent figures were returning to the Catholic Church. As a result, Protestants were increasingly on the defensive. The German princes split into two military camps based on religious affiliation: the Evangelical Union and the Catholic League.

In August 1619, a Bohemian diet elected as king the Protestant elector-prince of the Palatinate, Frederick V, and the conclave of elector-princes elected Ferdinand II (r. 1619-37) Holy Roman Emperor. On November 8, 1620, a force combining troops from the Catholic League and the imperial army decisively defeated Frederick V's largely mercenary force at the Battle of White Mountain. Throughout the 1620s, the combined imperial and Catholic forces maintained the offensive in Germany, enabling Ferdinand to establish his authority in the Hereditary Lands, Bohemia, and Hungary.

Equating Protestantism with disloyalty, Ferdinand imposed religious restrictions throughout the Hereditary Lands. In 1627 he implemented a long-planned decree to make Bohemia a one-confession state: Protestants were given six months to convert or leave the country. In the face of a strong Hungarian nationalist movement headed by the Calvinist prince of Transylvania, however, Ferdinand could maintain his hold on Royal Hungary only by confirming guarantees of religious freedom.

Foreign intervention by Denmark, Sweden, and France kept Ferdinand from bringing the war to a conclusion through military power and also frustrated his efforts in the mid-1630s to reach a compromise with the Protestant German princes. The subsequent military campaigns of the Thirty Years' War, however, only marginally affected those portions of the Habsburg territories that are part of modern Austria.