Between 1962 and 1965, there were several meetings between NVN and China on the degree of China's military involvement in the Vietnam War. In May 1963, Liu Shaoqi told Ho Chi Minh that while Ho could count on China for support, such support would be that of a 'strategic rear', whatever that meant, and that China's support would be limited, whatever 'limited' mean would be up to China. There was also the on-going Sino-Soviet split at the same time and some PLA generals worried that there could be war between the Soviet Union and China. Mao could not ignore this potentiality and despite his assurance to Ho in Apr 1965 that China would escalate involvement to match America's, two crucial events made China backed off that promise.
The first crucial event was Ho's insistent on neutrality regarding the Sino-Soviet split. Mao knew that members of the NVN's Politburo were advocating that Ho should make overtures to the Soviets.
The second crucial event was after Mao and Ho negotiated some degree of Chinese military involvement where Chinese 'advisors' would build roads and assorted infrastructures and provide anti-aircraft direct support, meaning it would Chinese gunners who would be doing the shooting. The NVN's Politburo was overwhelming opposed to any Chinese military activities that would involve weapons controls. The Viet Minh was willing to put up with Chinese 'advisors' during the fight against France, but not to the degree that there would be active Chinese combat engineers battalions on Vietnamese soil. Mao conceded that North Vietnamese pilots could launch and retreat to Chinese air bases to protect them from American fighters.
Upon arrival in NVN, there were tensions between the two armies. The Chinese were better equipped and armed so there were many instances of theft by the inferior NVN troops. In return, the Chinese troops became abusive and that raised the memories of many who remembered how Chinese troops behaved towards Vietnamese peasants during the disastrous land reforms of the 1950s. Ho wanted the US to know that NVN could wield an effective air defense and that it was with Chinese help. He ordered their use at US bombers before the Chinese engineers could secure and shelter their guns. The result was that when the Americans responded, there were both high Chinese and North Vietnamese casualties as well as much destruction of valuable anti-aircraft batteries of all types.
After this disaster and Ho's ambivalence towards China regarding the Sino-Soviet split, China no longer wanted to involve China in any direct confrontation with the US military even if the SVN/US alliance cross the 17th parallel. China will continue to provide arms, training, and rear support, but not in direct combat actions. China gambled correctly that the SVN/US alliance was not interested in unification but partition and that made rhetorics credible.
