The Soviet Union declared war on Japan on 8 August 1945, just before Japan’s surrender announcement.
Soviet Foreign Minister Vyacheslav Molotov informed Japan late on 8 August that the Soviet Union considered itself at war. Soviet forces then invaded Japanese-held Manchuria shortly after midnight on 9 August.
The declaration violated the Soviet-Japanese Neutrality Pact, but Soviet leaders had promised the Western Allies that they would enter the war against Japan after Germany’s defeat. The attack opened a major front in northeast Asia and destroyed Japanese hopes that the Soviet Union might mediate a favorable settlement.
Some sources describe the event as occurring on 9 August because the invasion and news reached Tokyo after midnight in Japan Standard Time. The declaration itself was delivered late on 8 August Moscow time.