Here's an idea: I've seen variations on the Pearl Harbor attack, on timing and damage, but what if there had never been an attack at all? It's the Day of Infamy that finally sparked the US into actively and effectively joining into WW2, but what if that hadn't happened?
I leave it up to speculation whether the Japanese would have gone after English or Russian outposts, that's a whole subject in itself.
The Isolationist movement was still very strong in the US, even for all our aid to England; without such an active act of aggression how long would we have remained "officially neutral". How long would Great Britain have held on without our full commitment? What reason, if any, would have been given should the US finally join the active war, assuming we would at all?