World War I began in 1914 and lasted until 1918. It was caused by increasing imperialism, militarism, and nationalism in Europe, which led countries to form secret alliances. The war started when Archduke Franz Ferdinand of Austria was assassinated by a Serbian nationalist. This caused Austria-Hungary to declare war on Serbia and triggered the web of alliances that drew other European powers into the conflict on two sides - the Allied powers including France, Britain and Russia, versus the Central powers of Germany and Austria-Hungary. The U.S. initially remained neutral but later joined the Allied side in 1917 after German U-boats sank American ships and the Zimmerman Telegram proposed a German alliance with Mexico