At the end of the war Japanese military and civilian leaders considered two strategies to force an acceptable end of the conflict. First, they could ask the Soviet Union, at that point not engaged in the War in the Pacific, to mediate terms of surrender between Japan and the Allies. Second, Japan could fight a final defence of the Home Islands so costly it would force the United States into negotiation. Both of those plans changed drastically when on 9 August the USSR entered the war on the side of the Allies. After quickly sweeping up Japanese units in Manchuria, the Soviets were poised to attack Japan from the North. The USSR entering the war and crushing Japanese forces in Manchuria so swiftly meant both options favoured by Japanese leaders were no longer viable. Japan could not ask the USSR to mediate a surrender if they were a party in the conflict. The military could not fight a two front invasion, particularly given how vulnerable the northern Home Islands were given the concentration on defence against American attack to the South. Soviet intervention in the conflict rally brought an end to the war. Bombing Hiroshima and Nagasaki was not required to end the conflict and so not morally justified.
[P1] Japanese civilian and military leaders are key actors. [P2] By August the only options Japan had left involved the USSR staying out of war. [P3] When Japanese leaders realised they had no more options left they surrendered.