The USSR and Japan signed a non-aggression pact shortly before German invasion of the USSR. Both sides kept the agreement until almost the very end of the war, as it was not in either's case to engage in hostilities. In the case of USSR, after Germany invaded in 1941, they need all of their resources to fight in the European theater. Declaring a war on Japan not only would have forced them to fight on two fronts, but also made their eastern ports unusable - and those ports were critical for receiving aid form the United States.

At the Yalta Conference in 1944, Stalin agreed to assist the allies by declaring war on Japan within 90 days of the conclusion of the war in Europe - a commitment that Stalin kept.