The Soviet -- Japanese War (Russian: Советско - японская война; Japanese: ソ連 対日 参戦, "Soviet Union entry into war against Japan '') was a military conflict within the Second World War beginning soon after midnight on August 9, 1945, with the Soviet invasion of the Japanese puppet state of Manchukuo. The Soviets and Mongolians terminated Japanese control of Manchukuo, Mengjiang (Inner Mongolia), northern Korea, Karafuto, and the Chishima Islands. The defeat of Japan 's Kwantung Army helped in the Japanese surrender and the termination of World War II. The Soviet entry into the war was a significant factor in the Japanese government 's decision to surrender unconditionally, as it made apparent the Soviet Union would no longer be willing to act as a third party in negotiating an end to hostilities on conditional terms.
when did the soviets declared war on japan
August 9, 1945