Kissinger was Richard Nixon's most trusted advisor on foreign affairs. He was in government during the Cold War and promoted what he called "realpolitik" in dealing with the Soviet Union and Communist China. He was a major force behind the 1973 ceasefire in the Vietnam War. He won a Nobel Peace Prize for the ceasefire, but the agreement fell apart in 1975. Under Kissinger, the US opened up relations to China, which is considered one of his biggest successes. He also supported détente, an easing of the rivalry with the Soviets.