On what date did the Soviet Union declare war on Japan in 1945?

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

Soviet Foreign Minister Vyacheslav Molotov informed Japan's ambassador in Moscow on August 8 that the Soviet Union would enter the war. The declaration took effect from August 9, when Soviet forces began their invasion of Japanese-held Manchuria. The timing fulfilled a commitment made at the Yalta Conference to join the Pacific war within three months of Germany's defeat.

The Soviet offensive struck Japanese forces across several fronts in Manchuria. Soviet troops also attacked Japanese positions in southern Sakhalin and the Kuril Islands. Japan had hoped the Soviet Union might mediate with the Western Allies, so the declaration destroyed an important diplomatic expectation.

The Soviet entry came between the atomic bombings of Hiroshima and Nagasaki and added military and political pressure on Japan's leaders. Historians continue to debate the relative importance of the atomic attacks and the Soviet invasion in Japan's decision to surrender, but the dates and sequence of events are firmly documented.