The cause of the battle was a revolt by a group of Czech lords who threw two governors and a secretary out of the windows of Prague Castle. This incident is known as the Defenestration of Prague. They quickly built up an army of 30,000 to make an independent Protestant country. The Czechs were defeated by a well trained Imperial army, the battle lasting less than one hour. Emperor Ferdinand II replaced Frederick as King of Bohemia, and then killed many who had opposed him. Others were given a choice, convert to Catholicism or leave the country.