Answer:
While you cannot exactly know what would have happened if the United States had not entered the war, one can make guesses. U.S. loans to the Allies were vital to the war as early as 1915. US. entry into the war in 1917 was important in maintaining the morale of the Allies especially France. U.S. forces were quite vital in the Allied offensives in the summer and fall of 1917.
