China to push their own territorial claims against the Soviet Union. The Soviet Union used the islands as part of an antisubmarine warfare network guarding the mouth of the Sea of Okhotsk. During the first half of the 1950s, other unsettled problems included Japanese fishing rights in the Sea of Okhotsk and off the coast of the Soviet maritime provinces and repatriation of Japanese prisoners of war, who were still being held in the Soviet Union. Negotiation of these issues broke down early in 1956 because of tension over territorial claims. Negotiations resumed, however, and the Soviet Union and Japan signed a Joint Declaration on October 19, 1956, providing for the restoration of diplomatic relations and ending the war. The two parties also agreed to continue negotiations for a peace treaty, including territorial issues. In addition, the Soviet Union pledged to support Japan for UN membership and waive all World War II reparations claims. The Joint Declaration was accompanied by a trade protocol that granted reciprocal most-favored-nation treatment and provided for the development of trade. Japan derived few apparent gains from the normalization of diplomatic relations. The second half of the 1950s saw an increase in cultural exchanges. Soviet propaganda, however, had little success in Japan, where it encountered a longstanding antipathy stemming from the Russo-Japanese rivalry in Korea, Manchuria, and China proper in the late nineteenth century, from the Russo-Japanese War of 1904–5; and from the Soviet declaration of war on Japan in the last days of World War II, in accordance with the Yalta agreement. The Soviet Union sought to induce Japan to abandon its territorial claims by alternating threats and persuasion. As early as 1956, it hinted at the possibility of considering the return of the Habomai Islands and Shikotan if Japan abandoned its alliance with the United States. In 1960, the Soviet government warned Japan against signing the Treaty of Mutual Cooperation and Security with the United States, and after the treaty was signed, declared that it would not hand over the Habomai Islands and Shikotan under any circumstances unless Japan abrogated the treaty forthwith. In 1964, the Soviet Union offered to return these islands if the United States ended its military presence on Okinawa and the main islands of Japan. 1960s–1975: Improving relations Despite divergence on the territorial question, on which neither side was prepared to give ground, Japan's relations with the Soviet Union improved