They don't have to go to war with China, they need only pass on intelligence to Vietnam sufficient for China to be defeated if conflict between them does occur. (Wasn't that what the Japanese did in 1979?)
Not every single engagement, we lost quite a few battles actually. 50th Army's 448th Regiment was devasted during the retreat, and 219 were captured by Vietnamese. That was the heaviest Chinese casualties sustained in a single battle since Korean War. Give credits where it's due, the Vietnamese fought very well and did caused us significant problems. In short, we underestimated them.
The military's performance in 1979 was far from a 90. Tactically it was poorly led and organized. The troops were under-equipped and suffering from lack of training due to Cultural Revolution. We didn't force Vietnam to withdraw from Cambodia and we never pushed deeper than 60km.
No, I'm not. I'm relying on my memory of a decades-old newspaper or newsmagazine report.
No, we were still smarting from the N. Vietnamese absorption of S. Vietnam, otherwise the U.S. would not have protested Vietnam's invasion of Cambodia to depose the genocidal Pol Pot regime.
While the U.S. had no bone to pick with China, the Japanese and Russians did. As GRU defector "Victor Suvorov" wrote, the Soviets had an extensive intelligence network in Japan throughout the Cold War. Powerful enough to get Japanese agents to do their bidding at times, maybe even with Japanese gov't connivance. Just a small matter of Russia providing info to a Japanese agent who then passed it on to Vietnam. (Doing it that way prevented a direct confrontation between China and Russia, I guess.)
Russia yes, China no. The Chinese referred to themselves as North Vietnam's "reliable rear area" - that is, they were ideologically sympathetic but as a State matter committed to never invade or provide direct support of the North Vietnamese the way they did with the North Koreans. In his memoirs Kissinger records that for some years before hostilities between Vietnam and the U.S. cease the Chinese were building a road through China and Laos near the Vietnamese border; it was some time before U.S. analysts realized its purpose was to defend against or facilitate hostilities with North Vietnam. The Chinese realized that the Americans would not be staying forever and eventually they'd have to face the North Vietnamese alone.
