Basic Understanding of the Northern Territories Issue

Japan discovered and surveyed the Four Northern Islands before the Russians arrived there. By the early 19th century at the latest, Japan had effectively established control over the four islands. Russia’s influence had never extended toward the south beyond Uruppu Island. In 1855, the Treaty of Commerce, Navigation and Delimitation between Japan and Russia, which was concluded in a completely friendly and peaceful manner, confirmed the already established natural boundary between the islands of Etorofu (the northernmost island of the Northern Territories) and Urup Island. The Four Northern Islands had never been held by foreign countries.
However, nearing the end of the Second World War, on August 9, 1945, the Soviet Union, in violation of the Neutrality Pact that was still in force between Japan and the Soviet Union, opened the war with Japan. Even after Japan accepted the Potsdam Declaration, Soviet forces continued its offensive against Japan and occupied all of the Four Northern Islands from 28 August 1945 to 5 September 1945.
Subsequently, in 1946, the Soviet Union unilaterally incorporated the territories under occupation into its own territories without any legal grounds, and by 1948 had forcibly deported all Japanese residents of the Four Northern Islands (approximately 17,000 people).
Since then, up to the present time, the Soviet Union and Russia has continued to occupy the Northern Territories without any legal grounds. Due to the Northern Territories issue still not being resolved, Japan and Russia have not concluded peace treaty even after almost 70 years since the end of World War II.