The Soviet Union was the last major Allied power to declare war on Japan before its surrender.
The Soviet Union declared war on Japan on August 8, 1945, and Soviet forces began operations in Japanese-occupied Manchuria shortly afterward. The declaration came between the atomic bombings of Hiroshima and Nagasaki and Japan’s acceptance of the Potsdam Declaration.
The Soviet entry ended the Soviet–Japanese Neutrality Pact and opened a major front against Japan’s Kwantung Army. Soviet forces also attacked Japanese positions in areas including southern Sakhalin and the Kuril Islands.
The United States, the United Kingdom, and China had already been fighting Japan. Japan announced acceptance of the Allied terms on August 15, although the formal surrender was not signed until September 2.