Political map of the Arctic & the Far North 20 January 1925 (Soviet-Japanese Peace Treaty): The Japanese withdrew from the Russian mainland (Japan during the Siberian Intervention) in October 1922, allowing the Soviets to annex the Far Eastern Republic just weeks later. In December, Soviet Russia merged with its remaining satellite republics in Europe to form the Soviet Union (Treaty on the Creation of the USSR). Formal peace with Japan took another two years, but in 1925 the Japanese agreed to return North Sakhalin, its remaining occupation in Russia,  in exchange for economic concessions.