The Thirty Years' War was the culmination of the long struggle between Roman Catholics and Protestants that had resulted from the Protestant Reformation of the 16th century. The war was centered in the Holy Roman Empire. This unique political organization was at that time made up of Germany, including Bohemia and Austria, and the Spanish Netherlands (Belgium). The imperial title was held by the Hapsburgs of Austria, a Catholic dynasty. The Holy Roman Empire was supported politically by Spain, which was ruled by another branch of the Hapsburgs.
Some of the countries in the empire were under the direct rule of the Austrian or Spanish branch of the Hapsburg family. Most of the German states, however, were governed by rulers who were theoretically subject to the authority of the emperor but in reality had a large degree of independence. Many of the German states had Protestant rulers and chiefly Protestant populations. The Protestant states had become increasingly dissatisfied with the rule of the Hapsburgs, who had made frequent attempts to reduce Protestant religious liberties within the empire.