I know it's probably a bit ASB, as it would require an early PoD and big political changes, but I've had this thought, and I'm curious:
How would WWII have been affected if the United States joined Britain and France in declaring war in September 1939? For example, if they too had guaranteed the independence of Poland in a similar or identical declaration to that of the original Allies. And history somewhat repeats itself - the US joining their old ally France against a belligerent Germany. Again.
Obviously, I'm not proposing any changes in their military strength or readiness at this point, just their politics, so I'm assuming that their contribution for a year or two will be mostly in terms of production and supplies until their forces can really get going. Further out, I imagine US involvement in the European war will also affect Japanese policy one way or another.
What do people think?