In 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat.
The United States decision to join World War I prevented the defeat of the Allied forces in April, May and July of 1918. In August the influx of 2 million fresh American troops led to the collapse and defeat of the German forces. Without the American troops the Allies would most likely have lost the war.
