The United States declared war on April 6, 1917, after Germany declared unrestricted submarine warfare on all shipping, including ours. In a single month in 1917 Germany sunk 96 vessels, many of them from neutral countries. There were reports of even hospital ships sunk that were clearly marked with the red cross of the medical corps. The sinking of the Lusitania, the reports of German atrocities and America's growing sympathy for France all combined to overcome America's aversion to the war.
Meanwhile, Russia's withdrawal allowed Germany to move all its forces from the Eastern to the Western Front and the grip on Belgium and France did not appear weakened. A turning point came with the infusion of U. S. troops, filled with energy and animosity for alleged German atrocities; the course of the war finally turned in favor of the Allies.