>>723156361
They fought Russia in the first ww but they were on the side of winner back then. Losing to "inferior race" was one of the nails driven in to seal Tsar's fate. After Tsar was deposed of and the commie took over they've been biding their time and there were small skirmishes throughout the west coast but they were too occupied with Hitler to rally full force against Jap (one of the reason why Jap allied themselves with German even though German was a loser in ww1)

The Japs in turn were very paranoid about ruskies throughout ww2 and their HQ probably see being cut off war resources something urgent enough to warrant opening an eastern front, severely underestimating US by thinking that they can't recover after pearl harbour.