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Again, it is hard to say. What Shinpachi posted above is the most likely response (I think).
In the early 1970s China had no real ability to fight the US in a direct head-on manner - they were too far behind in terms of technology.
China had said at the time of the Korean War that they would not tolerate a 'Western Culture' (this is I think the correct translation for the terms the Chinese Ambassador used) nation directly on their border, and this is a major (the major?) reason for them sending in their army when the Allies reached the Yalu - they had actually warned the US and UN about their probable response. This attitude had not changed at the time of the Vietnam War. They had seen what occurred on the various fronts of the various modern wars over the past century and did not want to put themselves in such a potentially disadvantageous position. The Eastern European satellite/Warsaw Pact nations area were the Russian version of the same attitude. In effect, the USSR and China wanted a buffer zone between themselves and the most likely potential invaders.
Incidentally, this concept is still part of the Chinese rational involved in the Taiwan situation today.
Also maybe see my post in the Arc Light One discussion thread:
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Arc Light One"