I'm considering the merits of doing a TL where Imperial Japan decides after Pearl Harbor to also fight the Soviets by shutting down the lend lease picket to Vladivostok and invading Siberia. There isn't any POD beyond them just deciding to fight Stalin, too. It goes without saying that this is insane and that they will get ROFLstomped on land, but they certainly have the capacity to close down the Vladivostok route.
It would be a disastrous strategic move for Japan, but I honestly think the leadership was crazy enough to try something like this. They picked a fight with half the planet's population and a much greater proportion of its wealth and industrial base in OTL while lacking 1/2 the merchant capacity required to feed the Japanese population and best of all, planning to treat the fight against the European powers, their colonies, the United States, the settler states (Australia New Zealand, South Africa, etc.), and the OTL Cold War/Konfrontasi with the Soviets as a secondary front while they continued their war in China. By the time you do that, I think you're pretty much crazy enough to try anything.
My question is, am I reading the Imperial Japanese leadership correctly? Were they in fact crazy enough to just wake up on the wrong side of the bed one day and go "Hell, we're fighting everyone else, let's fight the USSR, too!" without any particular change in events from OTL?
If not, what POD would be required?