The islands of Etorofu and Kunashiri and the Shikotan and Habomai islets were occupied by the Soviet Union in 1945. According to Soviet claims, these islands are a part of the Kuril Islands, granted to the Soviet Union in the post-World War II Yalta and Potsdam Agreements, and the Russian government has upheld that policy. Japan maintains that they are an integral part of the Japanese home islands, illegally seized by the Soviet Union.
The two never signed a formal peace treaty after World War II because of the Kuril Islands dispute, say experts. The US has been a longtime backer of Japan’s claim to the territory.