The Soviet Union declared war on Japan on August 8, 1945, shortly before Japan surrendered.
The Soviet declaration followed a wartime promise made at the Yalta Conference to enter the war against Japan after Germany’s defeat. Soviet forces began military operations in Japanese-held Manchuria after midnight on August 9, according to local time in the region.
The Soviet offensive attacked Japanese forces in Manchuria, Korea, southern Sakhalin, and the Kuril Islands. Its entry created a major new front while Japan was already facing the atomic bombing of Hiroshima and the possibility of further attacks.
The declaration date can cause confusion because the Soviet announcement occurred in Moscow on August 8, while military operations began on August 9 in East Asia. The Soviet Union was therefore the final major Allied power to enter the war against Japan.