How did the United States’ decision to join the Allies affect World War I? The decision played a minor role because the United States was unable to mobilize troops fast enough to defeat Germany. The decision played a major role because the United States supplied thousands of men and supplies to aid in the Allied victory. The decision played a minor role because the United States sent most of its resources to protect its border with Mexico. The decision played a major role because the United States supplied millions of dollars and political pressure to aid in the Allied victory.

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.