Well, Japan lost WWII and treaties among the victors gave the Kurils to Russia.
> Although by the terms of Article (2c) of the 1951 San Francisco treaty, Japan renounced all rights to the Kuril Islands, the treaty did not apply to the islands of Kunashiri, Etorofu, Shikotan and Habomai since they are not included in the Kuril Islands. Also, the Soviet Union did not sign the San Francisco treaty.