Beijing resumed its seat in the United Nations in October 1971, and U.S. President Richard Nixon visited China in 1972, so a Sino-U.S. alliance against the Soviet Union emerged as a foreign strategy in China. Chairman Mao set forth the Three Worlds Theory in 1974, which, regarding the Soviet Union as another hegemonic superpower besides the United States, sought to “treat the Soviet and U.S. hegemonism differently,” that is, official doctrine was to oppose the hegemony of both superpowers, but to concentrate more on that of the Soviet Union.