Thirty Years' War
Bohemia was so angry about the Catholics it they decided to not make Ferdinand II the king of Bohemia anymore and but made Frederick V the king instead. However, because Matthias was dead and so Ferdinand was the next person waiting to be the emperor, Ferdinand was in charge of the whole Holy Roman Empire now. Some people also thought Frederick should not be allowed to be king of Bohemia. All of that made the Holy Roman Emperor decide to fight Bohemia.