The Soviet Union invaded Manchuria when it entered the war against Japan on August 9, 1945.
The invasion targeted Manchukuo, the Japanese puppet state established in Manchuria. Soviet forces attacked from several directions shortly after midnight, advancing with large numbers of troops, tanks, artillery pieces, and aircraft. The Japanese Kwantung Army, once a major force, was rapidly overwhelmed.
The Soviet declaration of war came on August 8 in Moscow time, and operations began on August 9. This timing placed the invasion between the American atomic bombings of Hiroshima and Nagasaki. Japan's leaders had hoped the Soviet Union might mediate a negotiated peace, so the Soviet attack destroyed an important diplomatic expectation as well as creating a new military threat.
The invasion, the atomic bombings, and the worsening military situation all contributed to Japan's decision to accept the Potsdam Declaration. The Kwantung Army officially surrendered on August 16, although fighting continued in some areas until the broader surrender process concluded.