After World War II, the USSR occupied the two southernmost Kurile Islands, plus two of the Hobomai islands off Hokkaido. The Kurile Islands were considered strategically significant because Japan's naval fleet left from there to attack Pearl Harbor (see Map 5). Coincidentally, President Truman's Order No. 1 gave Stalin permission to occupy and hold parts of Japan “proper.” By never signing a peace treaty, the modern-day Russian government still controls these four islands under the guise of occupying Japan “proper.”

The Kurile chain consists of 36 islands stretching from the southern tip of the Kamchatka Peninsula to the northeastern edge of Hokkaido Island. The two most southern of these—Kunashiri and Etorofu (or in Russian: Kunashir and Iturup)—were traditionally thought by the Japanese to be part of Hokkaido Island. Through a series of nineteenth-century Russo-Japanese treaties, the Kuriles fell under the ownership of Japan: in the Treaty of Shimoda (1855) and the Treaty of St. Petersburg (1875), Japan handed over its interests in Sakhalin Island to Russia in return for total control of the Kurile Islands. Following the 1904–1905 Russo-Japanese War, the 5 September 1905 Portsmouth Peace Treaty allowed Japan to regain sovereignty over the southern half of Sakhalin. With the opening of Soviet-American negotiations for Soviet entry into the Pacific War, however, the centuries-old dispute over the Kurile Islands reemerged. Through a lengthy series of negotiations and agreements—including the Cairo Declaration, the Teheran Conference and the Yalta agreement—Stalin wove a diplomatic web that allowed the USSR to reoccupy the islands.

The October 1943 Cairo Conference, which the USSR did not attend, first raised the question of post-war territorial acquisitions. On 27 November 1943, the Allies issued the following declaration: “The three great Allies are fighting this war to restrain and punish the aggression of Japan. They covet no gain for themselves and have no thought of territorial expansion. It is their purpose that […] Japan will also be expelled from all other territories which she has taken by violence and greed.”