I always figured that Hitler declared war on the US because he wanted to go after the US convoys supplying both the Brits and the Russians. The U boats took a terrible toll until we got convoys and ASW figured out. Had he been able to cut off that pipeline then things would have been questionable for both.
I’ve always felt that we should have supplied the Soviets just enough to keep them in the war but not enough to let them win. Let the Nazis and the Commies (two peas from the same pod) destroy each other. To borrow a quote from elsewhere “It’s a shame they couldn’t both lose”.
I just saw an interview that was done back in the ‘70s with Karl Doenitz. He said that he had asked for more Uboats during late 39 and 40. He told Hitler at that time that with more Uboats that he could force Britain to capitulate by cutting off the supplies they were receiving from the U.S. by convoy. He said that Hitler’s obsession with building a grandiose surface navy with large imposing battleships kept them from being able to finish off Great Britain when it was most vulnerable because Hitler was unwilling to build a large number of the relatively more inexpensive Uboats to finish the job on GB.
WW2 would likely have turned out a lot differently if Hitler had not been such a stubborn loon.