What major action did the Soviet Union take against Japan on August 8, 1945?

The Soviet Union declared war on Japan on August 8, 1945. Soviet forces began invading Japanese-held territory soon after midnight on August 9.

The declaration followed the atomic bombing of Hiroshima and came before Japan's surrender announcement. Soviet forces attacked Japanese positions in Manchuria, Korea, southern Sakhalin, and the Kuril Islands during the Soviet–Japanese War.

The Soviet move broke the Soviet–Japanese Neutrality Pact, which had been signed in 1941. Earlier Allied agreements, including commitments made at Yalta, had anticipated Soviet participation in the war against Japan.

The Soviet offensive added a major conventional threat as Japan's leaders debated surrender. Along with the atomic bombings and the Allied demand for unconditional surrender, it helped create the crisis that ended the Pacific War.