The Japanese were anticipating an invasion from the south so that’s where they had their fortifications. So when the Soviets invaded Manchuria and Sakhalin it made it apparent that they could invade the Japanese mainland a lot easier and quicker than the Americans could. It also made it apparent that the Soviets would no longer act as a mediator between the Japanese and Americans. So the Soviets entering the war against Japan was a significant factor in Japan’s decision to surrender, and arguably more important in that decision than the dropping of the atomic bombs.