It's more of a decision then it is in AAP40 alone.
I have multiple scenarios floating through my mind. Does Germany want to do Sealion? If so - they'll probably want the US to sit it out at least the first turn.
good point if germany is attempting sealion then j1 dow is very bad. but i assume that there will not be too many sealion attempts. (actually there will probably be 1 attempt for every gaming group, just becuase they will want to see if it works, however after several games i assume there will be very few sealion attempts)
What happens if the US only declares war on Japan?!? It has the option of declaring war on Germany/Italy but nothing makes it do so! I can see myself cruising down to Brazil to get replacement income for the Phillipines. Build up for that turn 3-4 push you mentioned in Europe/Africa but put most of the 80+ income into the Pacific theater. Have the USSR declare war as well and have a dogpile on Japan.
remember usa only gets an additional 30ipc for its at war bonus. and it is if usa is at war not 30ipc for at being at war with g and another 30ipc for being at war with j.
usa could spend all of its money in either theater in either a j1 dow or j2 dow, the question is if a j1 dow is going to happen all the time. i think it will.
What will Japan do if the USSR (moves before Japan), moves it's air force ominously to it's eastern borders? What will Germany do? Will you still go for the J3 India crush? A couple soviet tanks running around freeing China territories could cause havoc for Japan. Those lime green Chinese are amazingly resurgent. Options, options, options.
im not sure what japan will do in those scenarios. i cant wait to play the game. but i think from all i have read that they will declare war round 1.