On what date did the Soviet Union declare war on Japan during the 1945 surrender crisis?

The Soviet Union declared war on Japan on August 8, 1945, during the final surrender crisis of World War II.

Soviet Foreign Minister Vyacheslav Molotov informed Japan’s ambassador, Naotake Satō, in Moscow that the Soviet government considered itself at war with Japan from August 9. The declaration followed the Soviet commitment made at the Yalta Conference to enter the Pacific war.

Soviet forces crossed into Japanese-controlled Manchuria shortly after midnight on August 9, beginning a major offensive against the Kwantung Army. Soviet attacks also spread to areas including Korea, southern Sakhalin, and the Kuril Islands.

Japan’s leaders were already confronting the destruction of Hiroshima and the possibility of further atomic attacks. The Soviet entry removed Tokyo’s hope that Moscow might mediate a negotiated settlement and became one of the central pressures leading to surrender. Japan announced its decision publicly on August 15, while the formal instrument was signed on September 2.