I was brainstorming a bit about the possibility of a neutral US in WW2 and came up with a half-baked scenario.
Let's assume the americans take a neutral stance in all of ww2 (an earlier PoD like FDR being assassinated, aggravating the great depression and forcing the americans to commit to "america first" could work). Perhaps they send some supplies for the allies, but nothing beyond that.
The nazis start WW2 at about the same time as OTL. They invade Poland, then France, and then proceed to harass the british while planning for Operation Barbarossa. The nazis still don't manage to get Britain to surrender (partly because of Hitler's refusal to cease attacks and allow the british to surrender through internal pressure), or if they do, the british don't maintain neutrality for too long. the british employ help from their dominions and colonies in order to attempt to supply the lack of an US to back them. Come 1941, the germans attack the USSR.
Question is, with the allies mostly comprised only of the UK, its dominions, Free France, and the USSR, how differently does WW2 play out? Will the germans manage to deal a knockout blow to the USSR or will they lose momentum and be forced to retreat just like OTL? How much harassment will the japanese be able to deal against the USSR, unless Stalin reaches a compromise with them? How bloodier will the war be in its later stages? What happens to the Mediterranean and North African Fronts?