The Soviet Union declared war on Japan on 8 August 1945, immediately before invading Manchuria.
Soviet Foreign Minister Vyacheslav Molotov informed Japanese ambassador Naotake Satō in Moscow that the Soviet government would consider itself at war with Japan from 9 August. Soviet forces then began a large-scale offensive into Japanese-held Manchuria shortly after midnight.
The Soviet entry had been anticipated by the Allies. At the Yalta Conference in February 1945, Joseph Stalin had agreed that the Soviet Union would enter the war against Japan within three months of Germany’s defeat in Europe. The timing also fulfilled a commitment made during Allied wartime diplomacy.
The declaration came between the atomic bombings of Hiroshima and Nagasaki. Japan’s leaders therefore faced both a new military offensive from the north and the prospect of continued atomic attacks. The Soviet invasion rapidly overwhelmed the Kwantung Army and added major pressure during the final days before Japan’s surrender announcement.