Okay, so I was doing the right thing by doing Support Our Party in the US from the beginning (that got me to pretty good membership in America First, with some Silver Legion and Bund support). Then I probably hosed myself by accepting Axis membership before Germany was well built up. Might be a good time to wait to join the Axis until Italy does, so I can have Pact of Steel.
All the ground units I built, I built as reserves, so at least that wasn't the culprit. I didn't build much air or naval early, on the logic that once I got the army to a solid standardized organization, land unit builds would take a back seat to air and naval builds. In fact, I don't think I'd finished any new naval units by the time the USA joined the Allies, except for four landing craft and ongoing serial convoy/escort builds. The air units for a carrier hadn't even finished. I think 3 CLs had.
The logic on AA as the fourth unit in a division was not so much to fight off aircraft as deciding one more support brigade that would make the division a little tougher and more diverse. A third infantry would cost more IC and require more supply (having more bang for that buck, yes, but at a cost). I wanted to be able to cover larger areas of land while progressing across China and thus wanted more, smaller but still tough divisions. The other option was AT, which seemed less likely to come into play for its specialty and more costly to operate (though slightly). If there were non-motorized engineers, I might have chosen those, but there are not in vanilla. And while my interceptors can sometimes provide fighter cover in China, just as often they are out of range. Sometimes even the bombers are. If there were a more useful choice I'd take it, but I haven't yet found a more diverse, tough Japanese division for the time in which I need to build an enormous amount of support brigades if I am to be ready for war. I'm definitely open to new ideas; I don't think the 1 Art is disposable, but maybe there's a very smart substitute for the AA that I have not considered. If I could build marines that early, I might just use those and later plan to replace them, forming them into a bunch of their own divisions, but I can't. I could build mountain brigades that early, but those really have their day in China and not so much later on. Cavalry, I guess, split away later as armies of occupation.
As Japan, what has struck me most is the need to be a cheap SOB. I care about every point of supply I can save. I care about every leader in the proper place without waste. I care about .05 IC. I want to waste nothing, because they don't have all that much and there's a ton to do. I want every chunk of resources and every IC. I am not going to disband even the crummy antique subs--any naval unit that can fight even by throwing rocks can have a patrol job later on, spotting incoming danger just as the real-life picket boats were to do. I do not believe I can be inefficient and succeed. So if anything I am doing is very inefficient, and the AA choice might be the most questionable in that regard, I'm definitely going to pay attention.
Where I probably diverge most from conventional thought is on garrisons. I've never seen a garrison division give me a tough fight and I am not putting faith in them for anything but static VP defense. For that, a 'division' of 1 Gar/1 MP is sufficient. For defense of relatively secure areas, I build the same 3 Mil/1 AA I did playing Germany many times, two per port. If it's an insecure area, I reason, it needs defending by real combat divisions with real leaders. So Okinawa will get by with my militia divisions, but I'll probably pile a whole infantry corps in Truk, because I want to keep it. Same for Midway and Guam and Wake, which I have seen that the USA keeps wanting back (can't blame them).