The Soviet Union declared war on Japan on August 8, 1945, in the Surrender of Japan timeline.
The declaration followed secret Allied agreements under which the Soviet Union promised to enter the war against Japan after Germany’s defeat. Moscow announced the declaration late on August 8, while Soviet forces began attacking Japanese positions in Manchuria soon after midnight on August 9 in local East Asian time.
The timing matters because sources may describe the military offensive as beginning on August 9. The declaration itself is commonly dated August 8 in Moscow and many historical timelines, while the invasion began after midnight in the region.
The Soviet entry added a major new military threat to Japanese leaders. It came days after the atomic bombing of Hiroshima and on the same broad sequence of events as the bombing of Nagasaki. Japan’s leadership then moved toward accepting the Potsdam Declaration and announcing surrender.