Austria-Hungary was the main cause of World War One. On the 28th of June 1914, Bosnian assassin Gavrilo Princip murdered Austro-Hungarian Archduke Franz Ferdinand and his wife. Austria-Hungary retaliated and blamed Serbia for the death, announcing that if Serbia did not give them the assassin then Austria-Hungary would declare war on them... Of course, Serbia did not have the assassin in their country, meaning they had nothing they could do. Serbia sent a distress call to Russia, which made Austria-Hungary send one to Germany, firing up the war.