In World War II , one Soviet precondition for a declaration of war against Japan was an American recognition of Mongolian independence from China , and a recognition of Soviet interests in the Manchurian railways and Port Arthur ; these were agreed without Chinese representation or consent . The Soviet threat of seizing parts of Inner Mongolia induced China to recognize Outer Mongolia 's independence , provided that a referendum be held . The referendum took place on 20 October 1945 , with ( according to official numbers ) 100 % of the electorate voting for independence . After the establishment of the People 's Republic of China , both countries confirmed their mutual recognition on 6 October 1949 . In line with the concluding statement of the Yalta Conference and to receive the southern part of Sakhalin and the Kuril Islands after the defeat of Japan , Soviet Union immediately fought against Japan . Later , the USSR and the Republic of China signed the Treaty of Friendship and Alliance with the Republic of China on 14 August 1945 . However , the National Government of the Republic of China believed the Soviet Union violated the treaty because it obstructed the efforts of the Republic of China to re-establish national authority in Manchuria and aided the Communist Party of China by giving it weapons surrendered by the Japanese Imperial Army . Additionally , the Soviet Union handed territorial control to the Chinese Communist Party in that area against United States Marines aided military dispatchment and installation of Nationalist Government in North East China during the early period of Chinese Civil War after 1945
Yalta Conference
Yalta Conference
United Nations General Assembly Resolution 505