Slate (of all sites!) quotes approvingly a Quora answer to the question: “What Would Have Happened if Germany Had Invaded the U.S. During World War II?”
Invading the North American mainland can be safely left in the realm of bad Hollywood films. And that’s even today, with larger ships, jet cargo aircraft, and more people. While it makes for a great strategy, in the end, it’s just a nonstarter. Why?
The Germans had no forward base in the New World. If they had seized Iceland, any of the French protectorates in the Caribbean, or northern South America, then an invasion, while still a stretch, could have been conceivable. Without forward bases to deploy to and from, an invasion isn’t going to happen.
Consider that the Wehrmacht was winning while America was out of the war. One of the most idiotic things Hitler did was to declare war on the United States on Dec. 11, 1941. While the Wehrmacht was about to get thrashed in the Soviet Union, it could have stage-managed that into a negotiated settlement if it had chosen to. When the U.S. entered the war, it was all in, and Germany didn’t have the cards for that kind of bet. Invading North America would have simply brought the U.S. immediately into the war, with results that would have been more disastrous than they were.
And even if the Germans had landed a sizable force here, how where they going to be resupplied? Any such force would have been trapped here until it was defeated, destroyed, or retreated. The U.S. could play at the U-boat game, and the Germans would have needed open logistics lines to keep themselves supplied. Assuming that they were somehow able to move further inland, they still would need a corridor or corridors open to the ocean for supplies and retreat. Not seeing how that could have happened.
In addition, everybody had guns. One commonality among the nations conquered by Germany is that private firearms ownership was heavily restricted or simply banned. With no such restrictions here and given the fact that modern combined arms tactics were still in their infancy, it’s difficult to see how the Germans would have avoided taking heavy casualties. The Germans would have faced an armed force at least 10 times the size of their invasion force, who were also motivated to ensure that they (the Germans) would lose.