May 1939 The French and British Empires end up in a shooting war against Japan and not with Germany over Poland They sell Poland out like they did the Czech Republic .
How would the French and British Navies do against Japan ?
How long would the War last .
Japan would be toast! Quickly!
Japan has no modern fighter in production and won´t have for almost a year, the Allies have three or four. France has lots of heavy infantry, both have strong naval forces and good bases in the area and land routes into China. Ship your old rifles and M1897 guns to China and the IJA is in serious trouble.
While France and the UK vs. Japan is not as unmatched as the USA vs. Japan, Japan is in no position to take on two fully industralized major powers that don´t have to watch their backs.
Still, it´s a) highly unlikely France and the UK would trust Germany and b) even more unlikely Japan would dare make a move before France has fallen.