How you're going to supply and equip a billion men? You don't, not without serious space colonisation to make up the difference. Unless you have an easy way to get loads of material down from space since you probably aren't getting all of it here on Earth and heavily automated factories to process all that in. And don't forget space-based farming too (with lots of automation) to help make sure you get enough food. You also need to get the supplies to them, but since I'm handwaving things by pointing at space as the only solution (I only bring it up since it's pretty much the only way you might somehow make this work), you might as well use land, sea, air, and space to help logistics out--this is actually the easiest part, since we have many examples of how logistics of armies of millions of men work, and there isn't much reason to have more than a few million men in one general area.
Basically, with the level of realism and feasibility we're dealing with here, Japan might as well just be using Gundams (cheaper than arming and feeding a billion men!) instead. Maybe drop one/multiple of those space colonies I keep mentioning onto an enemy city too. Of course, you don't need to crash a space colony into Earth to get the effect of leveling a city and its surrounding. Japan would have ICBMs that do exactly that. And so would their enemy, meaning you don't need a billion man army (and neither do they), since any war can be assumed to end fairly quickly with the majority of the planet dead or dying from nuclear warfare.
Yes, Japan was very nationalistic, they believed they were the superior Asian race, but if they could use Japanization on her colonies, it may work, unless Japan resorts to Genocide on some of its colonies. Japan might encourage Japanese people to move to the other colonies for better business opportunities, spreading the culture further as well. If there are any other ways Japan could unite her colonies, let me know.
All OTL. Lots of Japanese moved to Korea and Manchukuo for both business and colonisation. And it wouldn't be Imperial Japan without some mass murder here and there.
Sorry if there was confusion, I meant in wartime would Japan have the capacity to bring a billion men into arms, say in a war against the Reich. I'm not saying Japan is winning alone, it's with the help of the other Axis powers of Germany, and Italy. I was thinking Japanization over large populations like India, or China. Maybe they could do Divide and conquer like the Mongols in India, and give the majority Hindus more rights than the Muslim Bengalis, and Pakistanis, giving the Hindus a favorable opinion of the Japanese, and if Japan can lift Indians out of poverty with construction projects like Railroads, then India might stay loyal. I don't know how to get the Chinese to be loyal, but you could come up with something. I could see Japan becoming more Liberal over time though with enough time, especially if they pull a United States of Greater Austria.
The British built tons of railroads in India, many of which are still in use today. Didn't stop Indians from demanding the British leave. And any Co-Prosperity Sphere which is liberal and genuinely fair to its members is going to be centered around China before long (even counting Manchukuo and Korea) and any Japanizing idea rendered completely hopeless.
That's completely crazy. A United Earth right now could not muster a billion men under arms. That's one seventh of the world population. I'm pretty confident having one seventh of your entire population fighting in an army never happened, much less in a modern war where you also need to provide weapons and ammunitions to your soldiers (which also need to be produced - hard when everyone is off fighting). And finally in a Reich victory at best the Japanese crush the Chinese, keep Indonesia and Indochina and maybe take Australia and New Zealand. There's no way they take India or California, and I'm very doubtful about South America as well.
And in everyone of these conquests they will be fighting an extremely bitter decolonization war which they simply can't win. Japanizing even a tiny part of all this is right out, and any progress they end up doing on that front will be reversed anyway as soon as these occupied countries become independant.
No, 1/7 of the population under arms has happened, it happened in Paraguay during the War of the Triple Alliance, for instance. Bulgaria's forces during the Balkan Wars/WWI came very close to 1/7 of their population (and about 1/3 of their men!) as did Finland in WWII and probably some other examples I'm mising. More relevantly, Japan themselves were planning on mobilising even more in preparation to defend against Operation Downfall. Granted, the quality of the majority of the units raised in such a manner is very poor from the training to the supplies and weapons.
But if you notice one thing in common with Paraguay, Bulgaria, Finland, and Japan in those years, they mobilised that many people because they were pressed to their limit and up against far stronger opponents with far more resources.
