Likely a war with Japan would
be a 2nd great war given the size and military might of the two countries involved. Also in such a war it most likely becomes a "let's all get the Japs" affair
as every major European country piles on. (Does any European country have good relations with Japan?) Japan's allies might join in only to be destroyed along with Japan, as I don't think even they would be able to stand up to US, British Commonwealth, Germany, Russia, China, and possibly France and Italy.
Anyway, that's my take on the situation. Probably won't happen but it would certainly be interesting.