I wouldnt want to limit the US to much so they can respond to an over agressive Germany or Japan that may directly threaten the US secuirty. By this i dont mean Germany invading the UK as the US had no intention of joining the war even if the UK was invaded as defeat of the UK would follow quickly if germany got ashore.
Any conquests of Islands such as bermuda or Greenland, any invasion of canada, any south amrerican countries joining the axis before Dec 7th 1941. If Japan joined the war early against the UK then yes i would think the US would join as it would directly threaten its intrests in the pacific. As for Germany taking unhistoric contries such as Sweden, Turky or even overunning the Middle east, the US would not join the war as the US had no intrests in these areas at the time. You have to remeber the US public did not want anything to do with European affairs. They viewed Japan as more a threat to there intrests then Germany.
So unless the axis do something which could direclty lead to an attack or threaten the US directly then they would not enter the war. It is a much better game if the US does wait till dec 1941 as they can then have a fun war vs the Japs and also preasure Germany while there deep inside russia. An early war only runins the game for Japan as they have basicly no chance then, and might prevent germany from invading the USSR if it is prior to june 22nd 1941.
Also making any rules about the US joining the war if uk is invaded and same with the USSR justl imits the game to much. It should be a big chance for Germany to atleast attempt an invasion if they want to. If i was playing the UK i would also want to have the fun of fighting of a German invasion without the help of the USSR or USA. On that note it should be nearly impossible for Germany to invade the UK with a human player if there played correctly.
