The Soviet Union declared war on Japan on 8 August 1945.
Soviet Foreign Minister Vyacheslav Molotov informed Japanese ambassador Naotake Satō in Moscow on 8 August that the Soviet Union would consider itself at war with Japan from the following day. Soviet forces began their offensive in Japanese-held Manchuria just after midnight on 9 August.
The declaration fulfilled a commitment connected to the Yalta arrangements, under which the Soviet Union agreed to enter the war against Japan after Germany’s defeat. Its timing came between the American atomic bombing of Hiroshima on 6 August and the bombing of Nagasaki on 9 August.
The Soviet offensive rapidly attacked Japanese forces across Manchuria and also spread to areas including southern Sakhalin and the Kuril Islands. The declaration is sometimes dated 9 August because that was when combat began in East Asia, but the diplomatic declaration itself was made on 8 August in Moscow time.