The Soviet Union declared war on Japan on August 8, 1945.
The declaration took effect on August 9, when Soviet forces began invading Japanese-occupied Manchuria. This campaign is known as the Soviet–Japanese War and included operations in Manchuria, Korea, southern Sakhalin, and the Kuril Islands.
The Soviet move followed the Allied commitment made at the Yalta Conference to enter the war against Japan after Germany’s defeat. Japan had previously maintained a neutrality pact with the Soviet Union, but Moscow ended that arrangement before entering the conflict.
The Soviet declaration came between the atomic bombings of Hiroshima and Nagasaki and Japan’s surrender announcement. It added a major new military front at a moment when Japanese leaders were deciding whether to accept the Potsdam Declaration.