The Johnson administration sent U.S. combat troops to South Vietnam in 1965 in an effort to prevent the expansion of communism in Southeast Asia. Washington’s primary concern, however, was not Moscow but Beijing. By the mid-1960s, U.S. officials viewed the Soviet Union as an essentially conservative power, more concerned with protecting its vast empire than with expanding its borders. In fact, U.S. policy makers periodically sought Soviet assistance in achieving a peaceful settlement of the Vietnam War. As long as Khrushchev was in power, they found a receptive ear in Moscow. Khrushchev had sternly advised the North Vietnamese against a resumption of revolutionary war in South Vietnam. After October 1964, when Khrushchev was replaced by a new leadership headed by party chief Leonid Brezhnev (1906 –1982) and Prime Minister Alexei Kosygin (1904 –1980), Soviet attitudes about Vietnam became more ambivalent. On the one hand, the new Soviet leadership had no desire to see the Vietnam conflict poison relations between the great powers. On the other hand, Moscow was anxious to demonstrate its support for the North Vietnamese to deflect Chinese charges that the Soviet Union had betrayed the interests of the oppressed peoples of the world. As a result, Soviet officials voiced sympathy for the U.S. predicament in Vietnam but put no pressure on their allies to bring an end to the war. Indeed, the Soviets became Hanoi’s main supplier of advanced military equipment in the final years of the war. Still, Brezhnev and Kosygin continued to pursue the Khrushchev line of peaceful coexistence with the West and adopted a generally cautious posture in foreign affairs. By the early 1970s, a new age in Soviet-American relations had emerged, often referred to by the French term détente, meaning a reduction of tensions between the two sides. One symbol of the new relationship was the Antiballistic Missile (ABM) Treaty, often called SALT I (for Strategic Arms Limitation Talks), signed in 1972, in which the two nations agreed to limit their missile systems. Washington’s objective in pursuing such a treaty was to make it unprofitable for either superpower to believe that it could win a nuclear exchange by launching a preemptive strike against the other. U.S. officials believed that a policy of “equivalence,” in which there was a roughly equal power balance on each side, was the best way to avoid a nuclear confrontation. Détente was pursued in other ways as well. When President Nixon took office in 1969, he sought to increase trade and cultural contacts with the Soviet Union. His purpose was to set up a series of “linkages” in U.S.-Soviet relations that would persuade Moscow of the economic and social benefits of maintaining good relations with the West. A symbol of that new relationship was the Helsinki Agreement of 1975. Signed by the United States, Canada, and all European nations on both sides of the Iron Curtain, these accords recognized all borders in Central and Eastern Europe established since the end of World War II, thereby formally acknowledging for the first time the Soviet sphere of influence. The Helsinki Agreement also committed the signatory powers to recognize and protect the human rights of their citizens, a clear effort by the Western states to improve the performance of the Soviet Union and its allies in that area.