What are the causes of the Thirty Years' War?

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.