The Thirty Years War was an exceptionally bloody war that was ostensibly fought over religion; however politics was a more important factor.
There were growing political and religious tensions between the Protestant and Catholic princes of the Holy Roman Empire. In 1608, the Protestant Princes formed the Protestant Union; in response the Catholic Princes formed the Catholic League in 1609. Each side vowed that there would be no territorial gain by the other.
On May 23, 1618, a meeting was held in Prague between Protestant Officials and officials of the Holy Roman Emperor who were Catholic. The dispute became heated and the Protestant delegates ended up pushing the Catholics out a window where they fell seventy feet into a moat filled with latrine waste. This "defenestration of Prague" was the immediate cause of the declaration of war; but the causes were the ongoing religious and political rivalries in the Empire.