There were many territorial, dynastic, and religious issues that figured in the outbreak and conduct of the war. The extent of religious motives is debated, but cannot be dismissed, particularly in explaining individual behavior. Throughout the war there were shifting alliances and local peace treaties. The war as a whole may be considered a struggle of German Protestant princes and foreign powers (France, Sweden, Denmark, England, the United Provinces) against the unity and power of the Holy Roman Empire as represented by the Hapsburgs, allied with the Catholic princes, and against the Hapsburgs themselves.
The war began with the resistance and eventual revolt of Protestant nobles in Bohemia, which was under Hapsburg domination, against the Catholic king Ferdinand (later Holy Roman Emperor Ferdinand II). It spread through Europe because of the constitutional frailty of the Holy Roman Empire, the inability of the German states to act in concert, and the ambitions of other European powers.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.