The Soviet assault on Manchuria on August 9 1945 was agreed to by the Western allies and the Soviets at the Tehran Conference of 1943 and again at the Yalta Conference in Feb. 1945. The Soviets agreed to attach the Japanese in Manchuria within 3 months of the end of the war against the Germans. The Soviets lived up to the agreement. Their attach against the Japanese started exactly 3 months after the Germans surrendered.
Maybe the most crucial thing about dropping the Bomb, it ultimately saved Japan from being divided, like Germany and Korea.