ww1 By gabrielsltasko 94 Assassination of Archduke Franz Ferdinand Jul 28, 1914 the government of Austria-Hungary declared war on Serbia Aug 4, 1914 reat Britain declares war on Germany May 7, 1915 the German U-20 sank the British ocean liner Lusitania. Nov 7, 1915 Third Battle of Artois Jan 19, 1917 naval intelligence intercepted and decrypted a telegram sent by German Foreign Minister Arthur Zimmerman Apr 16, 1917 he United States formally declared war against Germany and entered the conflict in Europe Jun 25, 1917 Battle of Mount Ortigara Nov 11, 1918 The Allied powers signed a ceasefire agreement with Germany at Compiégne Jun 28, 1919 The Versailles Peace Treaty, signed on June 28, 1919, officially ended World War I.