The Thirty Years' War was one of Europe's bloodiest. It was fought in Germany, Bohemia, Hungary, and other Central European countries. The war began as a Protestant-Catholic battle but quickly became a power struggle between the Holy Roman Emperor and German lords. The Treaty of Westphalia established that the religion of the ruler is the religion of the state. This principle empowered each German ruler to select his state's religion, ending the Catholic Empire in Europe. The conflict gave German princes independence from the Holy Roman Emperor.
Both groups are called Slavs. In the years before World War I, Serbia wanted all Slavic peoples to form one empire, free from rule by the Turks or Austria-Hungary. The Serbs' wish to unite all the Slavs in one empire is an example of. imperialism.
