Assume that is not a WW-II in Europe (Hitler die in WW-I or in early 20s,and nazi don't keep the power in Germany,and Weimar in some way survive,or in Germany is a left wing government,but not insane).
Without a great war in Europe is very improbable that Japan in 1941 goes to war aganist the western powers.
But if a war in Asia and Pacific is inevitable,when the Japanese Empire alone is ready to strike the westerners?
Early 50s?
Later?