The main reason for wars in my roleplay would be that they'd need the men from other countries as resources to hold off their own countries extinction long enough for a permanent cure to be made. Nobody is just going to donate their men peacefully or else they'd be the ones who vanish from existence once they run out of people. They can't trust each other to share properly and there's not even enough men to share properly if they wanted to. Somebody is going to run out first and if you don't fight for them it's going to be you all out of luck. A Japanese Empire doesn't sound too ridiculous in that case other then the fact that they probably wouldn't want to be the aggressor and permanently take the countries over and become the bad guy, which could make things problematic once all is said and done. They'd have to believe they can come out on top during this time so that it doesn't matter what other countries think of them. There may be an axis power type alliance who will believe this, but if you don't think Japan would fit the bill then they will remain with the US.