The Soviet Union declared war on Japan on August 8, 1945.
The declaration took effect on August 9 in the region, when Soviet forces launched the invasion of Japanese-occupied Manchuria. The attack opened the Soviet–Japanese War, also called the Soviet invasion of Manchuria, and rapidly overwhelmed the Japanese Kwantung Army.
The Soviet move followed the commitment made at the Yalta Conference to enter the war against Japan after Germany’s defeat. It also ended the Soviet–Japanese Neutrality Pact, which had kept the two countries formally out of direct war with each other since 1941.
The Soviet declaration came between the atomic bombing of Hiroshima on August 6 and Japan’s announcement of surrender on August 15. Historians continue to debate the relative weight of the atomic bombings and the Soviet entry in Japan’s decision, but both developments dramatically changed the strategic situation.