The Allies won WW1 by simply outlasting the Central Powers. Nazi Germany was fighting a two front war for most of the conflict, an was bled by a long, grinding, style of trench warfare that took millions of her young men. When the U.S. entered the war in 1917 on the Allied side, it was new, with a huge industrial base already manufacturing war materiel, and a population unbloodied as of yet. There was no way Nazi Germany could have competed long term with America allied against her.
