Soviet forces invaded Manchuria when the Soviet-Japanese War began in August 1945.
The Soviet Union declared war on Japan on August 8, 1945, and launched its main offensive into Japanese-held Manchuria just after midnight on August 9. The region was defended by Japan’s Kwantung Army, which had once been considered an elite force but had been weakened by transfers to other fronts.
The Soviet offensive advanced rapidly across northeastern Asia. Soviet forces also attacked Japanese positions in southern Sakhalin and the Kuril Islands, while fighting spread across areas connected to Korea and China. The invasion placed additional pressure on Japan’s leaders during the same period as the atomic bombing of Nagasaki.
Manchuria was not Japan proper. It was the location of Manchukuo, a Japanese-controlled puppet state established in northeastern China in 1932. The Soviet attack is sometimes called Operation August Storm, although that name is especially associated with the broader strategic offensive. Japan announced its acceptance of surrender terms on August 15, while fighting continued in some areas afterward.