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 war 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, though even while neutral it became an important supplier of war materiel to the Allies. Eventually, after the sinking of American merchant ships by German submarines, the declaration by Germany that its navy would resume unrestricted attacks on neutral shipping, and the revelation that Germany was trying to incite Mexico to initiate war against the United States, the U.S. declared war on Germany on 6April 1917. Trained American forces did not begin arriving at the front in large numbers until mid-1918, but the American Expeditionary Force ultimately reached some two million troops.