I realize it's a game and not the most historically accurate, but I've been trying things in Hearts of Iron IV to help whet my imagination for "what if X did this instead?

If you play Japan, it's hard to put enough pressure on the Soviets if you go into Vladivostok into Siberia. There's just so little infrastructure and supply that there isn't a way to push quickly. A small enough army to not have supply issues, and it can't push. Big enough to push quickly or blitz and you quickly leave your supply lines behind.

There's also not a whole lot there that helps Japan and Japan really is hard pressed for resources, not just oil and rubber, but even just steel. Yes there is oil in Siberia now, but it wasn't discovered/developed in that era.

Rather than Siberia, it works better as Japan to attack the Soviets through Iran and then push into the Caucasus. It gets oil for you and your friends and takes most of the Soviets away, crippling them. Iran itself has a little developed, and the caucuses are the next best source after Texas.

Edit: can't reply since the thread is locked. In response to Masterzig, yeah I should have added attacking Iran only really works as Japan if you do it without getting bogged down in a war with China and stay at peace with Britain as long as possible (since British controlled Pakistan borders Iran). Take Iran, then just wait until Germany launches Barbarosa. You're right you aren't taking on the Soviet army by yourself, but it's enough to tip things toward forcing the Soviets to surrender. I don't see it being very realistic for the Axis and Japanese to cooperate that closely though - they never did IRL.

I don't know what Japan's landing craft capabilties were like. They captured many of China's ports while at war with them, so must have had something. Iran doesn't have much in 1936-1945, so invading Iran is pretty doable. I don't see any situation where Japan would have done that thoguh since they were just focused on thing nearer to them in the Pacific. Iran does have some oil fields though, which Japan really needed. Biggest issue is it's so mountainous it takes a while to get through to the Soviet border and British India/Pakistan is then in between your forces there and Japan. The oil fields Iran does have are enough to get Japan by for a while. And the politics (in the game at least) are such that nobody really cares if you take Iran in 1937.