I was in uniform back then. DXP originally thought he had scared the Soviets but the 1979 Soviet Invasion of Afghanistan was a wake up call China did not want to hear. It wasn't that Moscow was too scared to come to Hanoi's aide. It was because they were too busy getting ready for a war of their own.

Make no mistake, Brezhnev's invasion scared Deng. They invaded Afgahnistan by building roads into that mountainous country - in winter. It impressed me, a combat engineer. We couldn't do what the Soviets did with that speed and it showed Chinese reliance on terrain was no show stopper for a Soviet advance.

This showed that any Soviet obstacle breaching must be actively contested and not just at chokepoints. Something the PLA at the time did not consider.