Without the sacrifice of young US men in WWI, the German offensives would have succeeded in forcing a French mutiny. Without the US equipment and later manpower in WWII, the Soviet Union would have fallen, leading to an eventual peace settlement favorable to Germany. I believe that President Trump speaks the truth.
I read the other day that if the Marines had not come in at Belleau Wood, the Germans would have taken Paris. Some conjecture that would have caused the allies to lose.