If you built USA’s Pacific fleet, smash the invading fleet!
If not, congrats: you are facing a Polar Express (also called KAF). If Japan has a shuck of 4x4, builds ala IC and has another trannie to annoy Africa, you have two troubles: the first is defending Africa, because you cannot send more USA’s reinforcements until you repel the japanese invasion. You have two bad options here: left Karelia at german mercy (it can cause soviets colapse) or continue shucking guys to Karelia and ignore Africa (it can lead to axis economic advantage, not a good idea). You cannot spare even a single USA unit to Africa, because Japan will send 10 guys each round against them and have 1 bomber that can do annoying SBRs (with a income of 35, USA will struggle even to equal Japan, and WUSA has built limit of 10 anyway)
But that’s not your biggest trouble. If Japan stacks Wcan with a solid force (one you cannot attack without being whiped on the proccess), Japan not only can trade Cusa and Ecan. As you’ll buy units in WUSA, EUSA will be empty, so there’s a great chance of the full Japan’s stack in WCan advancing to Ecan next round (supported by tanks built in ala). You’ll have not enough income to defend EUSA unless you have plenty of tanks in WUSA, and even then, you’ll be forced to not buy in WUSA one round. Then, ECan stack will go back to Wcan and now menace taking that IC! The idea here is that you simply cannot defend both ICs at the same time. If you lose one, even for one round, you are toasted
I have seen fall WUSA (without possible retaking) before Moscow with that strat. In a game, I even managed take Whasington. Some times axis reaches so big income (because fall of Africa) that allies concede.
The only real good chance for allies in case of Polar Express is killing a big stack of germans in Eeu or Karelia, leaving them with so few forces that Germany can fall at the same time than any USA’s IC. But it’s not easy
The other choice, of course, is fighting for Pacific Ocean from the very begining (USA1). That makes you inmune to Polar Express and puts some IPC rich islands at your reach, as well of preventing the lost of Hawaii, Nzel and probably also Australia