Stalin had undertaken at Yalta to join the war against Japan three months after the surrender of its German ally, which meant August 9. Despite the 1941 non-aggression pact there was no warmth between Moscow and Tokyo, which had already fought a brief undeclared war in 1939: Japan was an Axis power and a signatory to the Anti-Comintern Pact, and also a menace to Soviet interests in the Far East after its seizure of Manchuria and invasion of China. Politically there was every reason for the USSR to extend its influence, just as the US was intent on getting to Japan first. With Germany defeated there was nothing to stop Soviet commanders from bringing overwhelming force to bear to ensure a decisive and prompt outcome, as they did.

The conflict lingered after VJ-Day because it took more than a week to get the ceasefire order to the Japanese armies in Manchuria: similar lags occurred on other fronts. The last units seem to have laid down their arms about a week before the formal surrender of September 2.