Which Soviet action on August 8, 1945, helped pressure Japan toward surrender?

The Soviet declaration of war on Japan on August 8, 1945, helped pressure Japan toward surrender.

Soviet Foreign Minister Vyacheslav Molotov informed Japan's ambassador in Moscow that the Soviet Union considered itself at war with Japan from August 9. Shortly afterward, Soviet forces invaded Japanese-held Manchuria, beginning the Soviet–Japanese War.

Japan had hoped that the Soviet Union might mediate between Tokyo and the Western Allies. The declaration destroyed that diplomatic strategy and opened a major front in northeast Asia. It came between the atomic bombings of Hiroshima on August 6 and Nagasaki on August 9, creating a rapidly worsening military and political crisis for Japan's leaders.

The Soviet declaration did not itself constitute Japan's surrender. Emperor Hirohito announced Japan's decision to accept the Allied terms on August 15, and the formal surrender was signed on September 2. The Soviet entry is nevertheless regarded as one of the decisive shocks that shaped Japan's final decision.