History: Atomic Bombing Of Hiroshima, Japan

The Soviet Foreign Minister Vyacheslav Molotov had informed Tokyo of the Soviet Union's unilateral abrogation of the Soviet–Japanese Neutrality Pact on April 5. At two minutes past midnight on August 9, Tokyo time, Soviet infantry, armor, and air forces had launched the Manchurian Strategic Offensive Operation. Four hours later, word reached Tokyo that the Soviet Union had declared war on Japan. The senior leadership of the Japanese Army began preparations to impose martial law on the nation, with the support of Minister of War Korechika Anami, in order to stop anyone attempting to make peace.
Responsibility for the timing of the second bombing was delegated to Colonel Tibbets as commander of the 509th Composite Group on Tinian. Scheduled for August 11 against Kokura, the raid was moved earlier by two days to avoid a five day period of bad weather forecast to begin on August 10. Three bomb pre-assemblies had been transported to Tinian, labeled F-31, F-32, and F-33 on their exteriors. On August 8, a dress rehearsal was conducted off Tinian by Maj. Charles Sweeney using Bockscar as the drop airplane. Assembly F-33 was expended testing the components and F-31 was designated for the August 9 mission.