Well, and the bombing. Germany put 30% of its entire war resources into stopping US/Brit bombers. By June 44, it was 40% of all aircraft just used as interceptors.
I did a study of Kursk if the US had not come in, and with 30% more air (especially the tank-busting Stukas) Germany would have won Kursk.
It’s easy to believe that Germany put that much into air defense. The casualty rate for the USAAF was higher than for infantry.