Which Allied power declared war on Japan on August 8, 1945?

The Soviet Union declared war on Japan on August 8, 1945. The declaration took effect at midnight on August 9 in the Soviet Far East, opening the Soviet–Japanese War against Japanese forces in Manchuria and other areas of East Asia.

The Soviet move followed the Allied demand for Japan’s surrender and came shortly after the atomic bombing of Hiroshima on August 6. Soviet forces quickly launched the Manchurian Strategic Offensive Operation against the Japanese-controlled puppet state of Manchukuo.

The declaration was especially significant because the Soviet Union had maintained a neutrality pact with Japan. That pact was still in force when the Soviet government announced it would no longer observe the agreement. The Soviet offensive, combined with the atomic bombings and Japan’s internal political crisis, helped accelerate the decision to surrender.