Protestant Union
In 1618 the largely Protestant estates of Bohemia rebelled against their Catholic King Ferdinand, triggering the outbreak of the Thirty Years' War. Frederick was asked to assume the crown of Bohemia. He accepted the offer and was crowned on 4 November 1619, as Frederick I. The estates chose Frederick because he was the leader of the Protestant Union, a military alliance founded by his father, and hoped for the support of Frederick's father-in-law, James VI of Scotland and I of England. However, James opposed the takeover of Bohemia from the Habsburgs and Frederick's allies in the Protestant Union failed to support him militarily by signing the Treaty of Ulm (1620). His brief reign as King of Bohemia ended with his defeat at the Battle of White Mountain on 8 November 1620 – a year and four days after his coronation.