According to Moscow time, the Soviet Union declared war on Japan on 8 August 1945.
Soviet Foreign Minister Vyacheslav Molotov informed Japan’s ambassador in Moscow late on 8 August that the Soviet Union was entering the war. The declaration took effect from midnight on 9 August in the Soviet Far East, creating a date difference that explains why some accounts give 9 August.
The Soviet attack followed a secret Allied commitment made at the Yalta Conference. Soviet forces invaded Japanese-held Manchuria, Korea, southern Sakhalin, and the Kuril Islands. The offensive rapidly overwhelmed the Japanese Kwantung Army and changed the strategic situation in East Asia.
The timing overlapped with the final crisis inside Japan. The United States had bombed Hiroshima on 6 August, and the Soviet invasion began as the Japanese leadership debated surrender. Historians continue to discuss the relative weight of the atomic bombings and Soviet entry in Japan’s decision.