The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917 (the Eastern Front, by contrast, was marked by much greater exchanges of territory). In 1915, Italy joined the Allied Powers and opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans. The United States initially remained neutral, although, by doing nothing to prevent the Allies from procuring American supplies whilst the Allied blockade effectively prevented the Germans from doing the same, the U.S. became an important supplier of war material to the Allies. Eventually, after the sinking of American merchant ships by German submarines, and the revelation that the Germans were trying to incite Mexico to make war on the United States, the U.S. declared war on Germany on 6 April 1917. Trained American forces would not begin arriving at the front in large numbers until mid-1918, but ultimately the American Expeditionary Force would reach some two million troops.