On 2 February 1946, the Presidium of the Supreme Soviet of the USSR issued two documents. According to the first decree “On Nationalisation of Land, Banks, Industrial and Communal Enterprises, Railway and Water Transport and Communication Facilities of the Southern half of Sakhalin Island and the Kuril Islands”, the Kuril Islands and part of Sakhalin became state property of the USSR, and their subsoil, forest and water became public property.

The second decree, “On the Establishment of the South Sakhalin Region within Khabarovsk Krai of the RSFSR”, established a new territorial unit with its centre in the town of Toyohara (now Yuzhno-Sakhalinsk).

On 29 January 1946, General Douglas MacArthur, Supreme Commander of the Allied Powers during the Allied occupation of Japan, the day before had issued a memorandum excluding all islands to the north of Hokkaido from Japanese state or administrative jurisdiction.

The adopted documents were the consequence of the surrender of Imperial Japan in World War II. At the Potsdam Conference of the leaders of the Big Three in the summer of 1945, the territorial terms of Japan's surrender were determined and Japan accepted them by signing the Japanese Instrument of Surrender.

Sources:

Collection of laws of the USSR and Decrees of the Presidium of the Supreme Soviet of the USSR for 1938–1956 (Moscow: State Publishing House of Legal Literature, 1956)

The Japanese Instrument of Surrender, dated 2 September 1945