When did the Soviet Union declare war on Japan during World War II?

The Soviet Union declared war on Japan on August 8, 1945.

Soviet Foreign Minister Vyacheslav Molotov informed Japanese ambassador Naotake Sato in Moscow that the Soviet Union considered itself at war with Japan from August 9. The announcement followed the Soviet commitment at Yalta to enter the Pacific conflict after Germany’s defeat.

Soviet forces began their invasion of Japanese-held Manchuria shortly after midnight on August 9, attacking from several directions. They also launched operations against Japanese positions in Korea, southern Sakhalin, and the Kuril Islands.

The timing is easy to confuse because the declaration was delivered on August 8 in Moscow while the invasion began on August 9 in the Far East. The Soviet entry into the war was one of the major shocks facing Japan’s leaders as they debated whether to accept the Potsdam Declaration.