It was after the US had left Vietnam, and by 1972 Nixon's trip to China had started to thaw relations between the 2 countries, while the US was trying to extract itself from Vietnam. So while the US might have thought that Communism was monolithic when the Vietnam War started, once the US got involved, US was reluctant just to pack up and go home all of a sudden, and abandon its allies.I remember that. Viewing it from the US, we had been in that strange Vietnamese War supposedly fighting Communism, and it was obvious from this that Communism was not monolithic. It also seemed to indicate how strong the Soviet Union was that China was aligned against it, but at the same time that there was a lot of resistance to Soviet world dominance.
I remember commentary at the time, that from the Chinese point of view this was similar to what imperial China traditionally did when less powerful neighboring states got out of line.
I can't help but think there is a lot of spin doctoring about the results of the Chinese invasion by Chinese sources, which is also the kind of thing imperial China did, up coming up with different explanations when things didn't always go their own way. The Chinese have a long history of lying too
They say that their actions were primarily motivated by fear of the Soviet Union, but it seems that it was really to help out their allies in Cambodia, which they failed to do. The Khmer Rouge lost power, and while they continued to exist for some years more, they never really had much control. There was no evidence that Vietnam threatened China, and while the Soviets helped Vietnam, Vietnam policies remained largely independent, they were nothing like the Warsaw back countries, and there was no need to "tie up" Vietnamese troops as claimed. Despite minor border issues and rivalry with the Soviet Union, I see no evidence that the Chinese felt truly threatened. The given explanation might have been just a face saving excuse for the fact that they failed in their primary mission, to save the the Khmer Rouge regime. If they just wanted to keep the Khmer Rouge alive to tie down Vietnamese forces, they didn't need an invasion for it, and the Khmer Rouge survived after the Chinese invasion.