1964-5 was not 1969, and LBJ was not Nixon. Nixon's threats (which, let's remember, even he never carried out in the face of opposition from the State and Defense Departments) were in the context of years of gradual escalation having failed (politically at least) and trying to salvage something by forcing Hanoi to bargain. In particular, in 1964-5 talk about nuclear weapons would be seen by the administration as Goldwaterite, reckless, allienating world opinion and especially US allies in Europe. Moreover, LBJ's fear that too drastic action could bring the Chinese in, so criticized for decades as unwarranted timidity, was actually justified:
"One common misperception has to do with Lyndon Johnson's handling of the war in the 1960s. Critics say that Johnson allowed his fear of China to impede his handling of the war. When he escalated the war in Vietnam, he gradually expanded the bombing from south to north as he was afraid of incurring China's wrath. Like the Chinese side, Johnson remembered the Korean War and wanted to avoid another confrontation. He remembered that during the Korean War, the U.S. had failed to heed Chinese warnings after MacArthur crossed the 38th parallel, thus triggering a clash with China.
"This time, Johnson and his advisers paid close attention to the Chinese role. They were afraid that if the United States pushed too hard or attacked North Vietnam without restraint, they would have a replay of the Korean War. Johnson's critics later said that China was just bluffing, that the Chinese weren't serious about intervening. Harry Summers and other military writers criticized Johnson for allowing his fear of Chinese intervention to undermine his bombing campaign.
"However, the new evidence from China suggests that Mao was seriously prepared to intervene. There was a secret agreement between Hanoi and Beijing that if the Americans launched a ground invasion of North Vietnam (at that time, the United States had restricted itself to a bombing campaign), China would send ground troops into North Vietnam and would not allow the United States to defeat Hanoi. If the Americans bombed North Vietnam, China would match the American military action by taking measures to protect North Vietnamese cities and to rebuild roads and bridges. They would also send anti-aircraft artillery units and army engineers to support North Vietnamese troops and help them deal with the air bombing pressure.
"Meanwhile back in China, Mao was making preparations in anticipation of war with the United States. He relocated industries, universities, and research institutions in the coastal areas of eastern China to the mountainous areas of southwest China. He ordered his people to build anti-air shelters throughout China.
"Mao himself had staked a lot on the outcome of the Vietnamese War in terms of security as well as ideology. Mao took the American escalation seriously; he interpreted it as a security threat. But he also believed that the success of North Vietnam had ideological significance. At that time Mao was criticizing the Soviet Union for not giving enough support to national liberation movements, for pursuing détente with the United States. Thus he hoped to use the Vietnam War as a way to embarrass Khrushchev -- to show him that China had closely befriended anti-imperialist movements of the Third World.
"For all these reasons, Mao was really interested in Vietnam and prepared to intervene. This means that critics of Johnson were wrong. The historical record shows that Johnson was prudent in his approach to the Vietnam War -- that he was right not to adopt more drastic measures. If the suggestions made by these critics had been adopted by Johnson, there would have been a real danger of war between the United States and China."
https://www.wilsoncenter.org/articl...ntially-to-vietnam-war-victory-claims-scholar
