So to be honest I was getting a little bit worried that the scenario was too easy and I was wondering if I was missing something. Then, Japan researched a war tech of some sort and now the Philippines are gone and it's everything I can do to try and contain them, so congrats on luring me into a false sense of confidence. Interestingly though, this war tech happened the same turn I managed to snag Iwo Jima. So now I have a southern front that is collapsing while I'm simultaneously in position to possibly invade Japan proper.
Some observations:
-The AI is idiots with aircraft carriers but giving them an attack rating means they're **really** stupid. A Japanese carrier attacked Hawaii and managed to sink itself along with 19 planes it was carrying.
-I still feel like I was able to prepare very well for this war. Such is the curse of knowing history and playing a historical game. I just wanted to make you aware though, that you "could" limit the US military unit production with lua until the war actually starts in a number of ways (example, you could check if a city has built a unit and then unbuild it, or you could keep resetting what a city is building to the capitalization equivalent if they try building a unit, or you could probably add a script to canBuildSettings with
@Prof. Garfield 's help that wouldn't actually allow the US to build their better weapons if they or Japan/Germany didn't have the techs that allow the U.S. to go to war).
This might really handicap an AI US, but you can use the "isHuman" check to only have it happen if the USA is player-controlled. I think that if the U.S. was restrained further prior to actually entering the war, you might have Japan run more rampant.
-On that note, while I like that you've tied the entry of nations into the war to techs rather than set dates (enhances replayability considerably) you might consider a random chance of Japan getting the "offensive" tech each turn if the U.S. already has it (you may have already done this but I'm assuming Japan just got around to researching it).
-I would reaffirm my suggestion to let a few cities in California build the U.S. naval fighters because after the Japanese attack you're limited to basically Hawaii being able to build them.
As usual for your scenarios I'm having a blast so well done!