President Richard Nixon's historic visit to the People's Republic of China in February 1972 marked a Sino-American rapprochement and the beginning of the route to normalization of relations. This came more than twenty years after mainland China was “lost” to the communists and, less than a year later in 1950, attacked American-led United Nations forces in Korea. Thereafter, a key tenet of U.S. Cold War strategy was to “contain” Communist China by means of bilateral alliances and military bases in East Asia, and to isolate it by severing trade, travel, and diplomatic contacts and refusing to recognize the communist regime. The next twenty years were characterized by American opposition to UN membership for mainland China, three crises in the Taiwan Straits, offensive rhetoric, threats of nuclear attack, and the fighting of a proxy war in Vietnam. In ending this hostile estrangement in 1972, Nixon thus executed a dramatic reversal of U.S. China policy. The U.S.–China rapprochement was the most significant strategic shift of the Cold War prior to 1989, more so than the Sino-Soviet split. As Nixon and his National Security Adviser Henry Kissinger claimed, the rapprochement “changed the world” by transforming a Cold War international system made up of two opposing ideological blocs into a tripolar one in which great-power foreign policy was conducted on the basis of “national interest” and power balancing.