If the USSR has gone GIE, then all Comintern countries, including Mongolia and Tannu Tuva, need to be taken out to end the war on the Eastern Front. Germany and Italy are not in a position to do that - so it will be up to Japan.
Of course, Italy (or Germany) could send troops through Japanese territory so that the Japanese player can concentrate on the USA.
Of course, Italy (or Germany) could send troops through Japanese territory so that the Japanese player can concentrate on the USA.