What did the Soviet Union do to Japan on August 8, 1945, shortly before Japan surrendered?

The Soviet Union declared war on Japan on August 8, 1945, shortly before Japan surrendered.

Soviet Foreign Minister Vyacheslav Molotov informed Japan's ambassador in Moscow that the Soviet government considered itself at war with Japan from August 9. Soviet forces then invaded Japanese-held Manchuria just after midnight local time. The offensive also spread into territories including southern Sakhalin and the Kuril Islands.

The Soviet move broke the Soviet–Japanese Neutrality Pact, which had been signed in 1941. Japanese leaders had hoped that Moscow might mediate with the Western Allies, so the declaration destroyed an important diplomatic strategy. It also created the prospect of Japan facing a Soviet invasion from the north as well as continued American attacks from the Pacific.

The Soviet declaration came three days after the atomic bombing of Hiroshima and on the same date as the bombing of Nagasaki. Historians continue to debate how much each event influenced Japan's decision, but both developments sharply intensified the pressure on Japan's leadership.