I think the US has two primary options, assuming the Japanese go for a heavy Calcutta push:
1. Blockade the Japanese mainland while simultaneously taking Korea and building an IC there. This way the US can start liberating Chinese provinces (allowing Chinese infantry to be placed also) and pushing down towards Shanghai. The continuous convoy disruptions coupled with strategic bombing raids of the major IC on Japan would limit Japan to it’s mainland minor ICs that it likely built on turns 1-2. I’d imagine that the Japanese player would see the writing on the wall and surrender, saving everyone a few hours.
2. US captures Caroline Islands. The Caroline Islands’ air/naval base lets you threaten Japan and the DEI and Hong Kong, if you decide to go that route. Depending on the Japanese player’s moves, I’d aim for taking back the DEI. This would obviously allow the UK/ANZACs to maintain the IPC bonuses and would force Japanese air assets away from the mainland fight and buy the UK some more time. If the Japanese build naval forces to counter the US fleet, all the better; more ships = less ground troops going after Calcutta. Additionally, Japan focusing on the US navy would allow the UK to possibly launch an offensive into southern China and help liberate some Chinese territories, which can be a nuisance for the Japanese land army as well.
Any suggestions/comments from people are definitely welcome!