Playing as USA: you have your FP on/in Manila IIRC. It's crucial to keep that city IMHO. I draft all over the place
if I can airlift units to hold key islands in the pacific. It takes a few starts to do this right though. Run a few and try out different tactics to get a decent idea how the AI 'thinks'. And watch out for all those bombers! They will go for cities with 1) other bombers 2) fighters that are not on air superiority 3) fighters that are on air superiority 4) other units including ships 5) improvements 6) to kill pop 7) just for the fun of it. You need lots of Flak (IIRC 4 is the optimal number) in each city and fighters on air superiority (you can build airfields (sacrifice a worker) or you can keep escorted carriers around) for better chances of interception. But again - watch out for bombers! It's painful to watch 20-30 raids blasting away at everything you have.
So the short answer is: BUILD UNITS!
PS! IIRC you have a decent amount of subs in the pacific as USA
Playing as Commonwealth: this is a bit different from USA. You need tons of artillery built down under and you need to ship them (can't be airlifted) to your FP + surrounding cities. Why artillery? Because the Japanese put a lot of strong units in cities close to your FP. Again you'll face bombers

It's easier to draft here since you're a monarch and not in democracy (draft rate per turn is only 1 for demo). I would build an airport and/or factories and/or power plants in major key cities to set them to max shield output early on. It's beneficial to sacrifice a few workers down under for airfields!
Build up your troops before you go for Japanese cities as both USA and Commonwealth.
You'll have to play defensive for a few turns.
Good luck - this scenario is very fun!
You can easily move up one or two levels when you play this.