What did the Soviet Union do on 8 August 1945 that increased pressure on Japan to surrender?

The Soviet Union declared war on Japan on 8 August 1945.

The declaration took effect shortly before Soviet forces invaded Japanese-held Manchuria after midnight on 9 August. This opened a major front against Japan’s Kwantung Army while Japan was already facing the destruction of Hiroshima and the threat of further Allied attacks.

Japan had hoped that the Soviet Union might mediate a negotiated settlement because the two countries still had a neutrality pact. The Soviet declaration ended that hope and removed Japan’s diplomatic buffer in northern Asia.

The Soviet entry into the war is often discussed alongside the atomic bombings as a major factor in Japan’s surrender. Emperor Hirohito referred to the Soviet entry when speaking to Japan’s military leadership, although his public broadcast emphasized the destructive power of the new bombs.