...or Czechoslovakia on VE-Day....
Like previous thread with similar topic name I'm interested in taking this for granted as a premise and then looking to a follow on question, "what would be the likely changes, if any, on the Soviet policy toward Japan?"
Do the Soviets join in the war with equal or greater eagerness and speed than OTL, or less?
Points in favor -
If there is a sense of underachievement in Europe and Germany in particular, securing northeast Asian gains may be seen as a form of compensation for the Soviets
The further eastern junction with the west in Europe might be the product of a war that ends earlier and that has spared the Soviets from some of OTL's losses, which makes the prospect of losses in the east less daunting.
The further eastern junction with the west in Europe might be accompanied by less allied progress in the Pacific, offering the Soviets the prospect of more dominant influence on the Asian mainland and a serious role in Japan itself.
Points against - There may not be an atomic bomb available promptly to help encourage the Soviets and accelerate everyone's rush to the endgame.
If western Allied advances in Europe came at the expense of advances in the Pacific, then that means the Soviets can anticipate it will be harder work to fight the Japanese compared to OTL.
The Soviets might delay and play hard to get, seeking to leverage the west's desire for support against Japan for concessions by the west in Europe, which the westerners may or may not entertain.
With the British, French, Americans and most Germans and some Poles in physical contact and capable, in theory at least, of joining forces against the USSR, Moscow may see protracted Japanese resistance to America as being in its own best interest to draw American focus and capability from Europe while the USSR seeks to repair and strengthen itself.