Japan is simple if you remember one basic truth: If germany is winning, and has taken moscow, leningrad, and stalingrad, then japan entering the war and triggering the collaboration government bonuses is often sufficient. All outcomes are beneficial to japan, but the best is to fight as long as possible and draw, for max army xp.
If you win, you get 1x 100% research. So their divisions will be. Starting out, put the nearest supply depot on 3x motorization.
This invasions should be planned out and ready to start as soon as the war is. The issue i have is. The ussr literally cannot push you back because they never bother to upgrade infrastructure east of the urals. I saw multiple old posts about invading usa early as japan so i thought i'd try it out.
That should put them at war with the allies, but it may not do so in time to save your position. It starts a 30 day countdown. Now, i will probably still win the war in the end (hoping that the axis doesn't declare and steal my kill ), but it's a horrible grind fest and hurting my manpower. Invasions should be from the kuril islands to alaska and the port just east of amori to seattle.
Therefore a winning strategy has three objectives, imo. In unmodded hoi4, the soviet union will at some point declare war on greece. On this run not long after germany surrendered and i was. Japan and the axis can win even if the us joins the allies as long as germany defeats the su.