Soviet forces invaded Manchukuo soon after declaring war on Japan in August 1945.
Manchukuo was a Japanese-controlled puppet state established in Manchuria in 1932. Japan installed Puyi, the last Qing emperor of China, as its nominal ruler. The state became a major base for Japanese military and economic activity in northeastern China.
The Soviet Union declared war on Japan late on 8 August 1945, and the invasion began shortly after midnight on 9 August. Soviet forces attacked Japanese positions across Manchuria, southern Sakhalin, and the Kuril Islands in a rapid campaign known as Operation August Storm.
The Soviet offensive added a major military crisis while Japan was already dealing with the atomic bombing of Hiroshima and the expected bombing of Nagasaki. Emperor Hirohito later cited the Soviet entry as an important reason for ending the war.
Manchukuo collapsed during the invasion, and Puyi was captured by Soviet forces before later being transferred to Chinese custody.