Sorry to derail the topic a little bit, but that's a very informative answer and a question arises - speaking of garrisoning, how would you handle defending China as Japan? I'm always in doubt on how to garrison that massive area of land. QUOTE]
Japan - MILITIA IS YOUR FRIEND!! Remember this statement, you will see it again as the wise gunny said to the recruit. Japan has to balance everything on the cheap, more to do than any major and less IC than any major to do it with. Before you even open your war with China, start pumping militia divisions of 3xMIL 1xART. I always shoot for about 16 of these divisions at start. They make great place holders to allow your line infantry to do the real fighting, are cheap and quick to make, use few officers, can actually fight well against many chinese units, and build up your practicals fast.
Assuming you take China out as a player should, manpower will never be an issue again. Your only concern is how to hold and expand. For holding China, I use 1 brigade of garrison/militia on each valuable province, and 2xCAV to play whack-a-mole on rebels. Break up some of your starting GAR divisions with LOW experience and make brigades to cover the rest. You can't do anything about revolt risk in China, Japan doesn't get an option of just occupying until we get to India and Siberia
. So all you want to do is keep control of the provinces you need and reduce supply costs as much as possible. Coastal ports that can be invaded by allies are covered by 3xGAR 1xAA divisions. These divisions make great islands garrisons also and can be shipped out as your conquests spread. I always assume whoever is invading a port will have aircraft assisting. A couple of AA brigades will chew them up. You could interchange them with 3xMIL 1x AA if yoyu want something that can actually walk to another province without a strat move. And always, I mean always, cover your mainland supply port/depot with serious defenders. Japan can be so easier castrated by the poorly structured supply system of the Pacific it isn't funny. This means a full division against the AI, a full corps against a player. NOTHING will ruin your day faster than losing your depot, game over.
Japan will lower it's construction practical fast. It has to improve ports, build airfields, build infrastructure, and needs coastal forts on islands, build them in major ports also. Depending on how the game is going you might even sneak in IC runs. I use forts of level 1 or 2 just to delay the invasion long enough for my fleets to show up and fight. Level 10 forts make an island almost invulnerable but Japan is wasting IC at that point on an island that can be bypassed. Any mainland port can be attacked from the sides ignoring the coastal forts.
Reaction forces depends on your situation. Assuming you still control the sea around China/Japan, keep Marines and line corps in Japan where they can be moved fast by transports covered by fleets to invaded areas. Once you lose control of the sea lanes, it is just a matter of time. Best you can do if that happens is post your corps in strategic areas and respond as best you can via strategic movement. If necessary call back line corps from the front lines wherever they are.
And remember, you have do all this while still building a fleet big enough to fight and defeat the Allies, expand your empire, and be ready to help AI Germany when it gets bogged down in Russia. No one ever said it would be easy.