The Soviet Union declared war on Japan on August 8, 1945.
Soviet Foreign Minister Vyacheslav Molotov informed Japan's ambassador in Moscow that the Soviet Union considered itself at war with Japan beginning August 9. Shortly after midnight on August 9 in the region, Soviet forces invaded Japanese-held Manchuria, beginning a major offensive against the Kwantung Army.
The declaration broke the Soviet-Japanese Neutrality Pact of 1941. Soviet leaders had promised the United States and Britain that they would enter the war against Japan after Germany's defeat. The Soviet attack opened fronts in Manchuria, northern Korea, southern Sakhalin, and the Kuril Islands.
Japan's leaders faced this attack alongside the atomic bombings of Hiroshima and Nagasaki. Historians continue to debate the relative weight of the Soviet entry and the atomic bombings in the final decision to surrender, but the timing is clear: the declaration came only days before the Japanese government's acceptance of Allied terms and before the public announcement.