That depends WHICH Americans. Here's how I envision he might have done it: Have a team hijack DC-3's at Gander and fly them into the Empire State and Chrysler buildings. Although New Yorkers would have initially been shocked and angry, after a year or so the America Firsters would be counseling surrender and accusing Roosevelt of "building a global empire."
The French suffered over 140,000 casualties trying to stop the Wehrmacht. Like the Poles before them, the French got to meet the German army at full strength. Hitler managed to waste 100 of his veteran divisions in Russia before we entered the war.