Pretty much everything has changed in U.S.-China relations since Richard Nixon and Premier Zhou Enlai signed the Shanghai Communiqué 40 years ago on February 21, 1972.
Nixon’s goals were purely geostrategic. By cultivating China, he sought most of all to put pressure on North Vietnam to come to terms to achieve his promised “peace with honor” there, hopefully before the November election. He also sought, through China, to pressure Moscow to embrace détente and thus put America in the catbird seat in relations with the two communist behemoths.
China had different priorities — most of all, Beijing wanted to stiffen President Nixon’s spine to oppose Soviet aggression and therefore to reduce Moscow’s threat to China.
“The U.S. nuclear umbrella is a principal reason why North Korea does not use its conventional forces to inflict a major strike on South Korea. That in turn reduces any South Korean temptation to get its own nuclear deterrent. But no first use would mean that the U.S. would not use nuclear weapons to counter a North Korean conventional attack, and so removes them as a reason — perhaps the principal reason — for the North to show restraint.”