World War I By chodges Jun 28, 1914 Assassination of Archduke Serbian nationalist kills the Archduke of Austria-Hungary. This is the immediate cause of WWI. Period: Sep 5, 1914 to Sep 12, 1914 First Battle of the Marne This battle on the Western Front stopped the German Schlieffen Plan. Apr 2, 1917 U.S. Entry in WWI Period: Jul 1, 1918 to Nov 1, 1918 Battle of the Somme Jun 28, 1919 Treaty of Versailles The Treaty was signed officially ending WWI.