World War I began in 1914 due to rising nationalism, imperialism, militarism, and systems of alliances between European powers. The immediate cause was the assassination of Archduke Franz Ferdinand by Serbian nationalists. The US initially remained neutral but was drawn into the war in 1917 after German submarine warfare threatened American merchant shipping and the Zimmerman Telegram proposed an alliance between Germany and Mexico against the US. The addition of American troops and resources helped the Allies defeat the Central Powers by 1918. Over 20 million people died making it one of the deadliest conflicts in history.