What was the historical contribution of the American expeditionary forces led by General John Pershing?
It tipped the balance of the war in favor of the French and English.
The United States officially entered World War 1 on April 6, 1917. But it wasn't until October 1917 that the 1st Infantry Division joined into the fighting as part of the AEF.
But the real build up did not happen until the Spring of 1918. By that time Germany was ready to call it quits anyway. But the French, in particular Marshall Petin, wanted unconditional surrender which the Germans found abhorrent and for good reason.
The fresh American troops provided just enough of a difference that the stalemate which had existed broke in favor of the allies. The only reason Germany continued the fight into November was it was attempting to appeal to the English and Americans for something other than an unconditional surrender. The Germans only gave in when they realized the hopelessness of their situation.