At the Yalta Conference ( February 1945 ) , the Allies had agreed that the USSR would take the southern part of the island of Sakhalin , which the Russian Empire had ceded to Japan in the Treaty of Portsmouth after the 1904 – 1905 Russo-Japanese War ( the Soviets already controlled the northern part ) ; and the Kuril Islands , which had been assigned to Japan in the 1875 Treaty of St. Petersburg . On the other hand , no agreement envisaged Soviet participation in the invasion of Japan itself
Yalta Conference
Yalta Conference
Operation Downfall