China and Vietnam Close to War

It's hard to believe 40 years after the war in Vietnam ended that these two would go to war.  Remember the NVA were being supported by China in their campaign to take South Vietnam which they eventually did with the fall of Saigon.  So here we are 40 years later and now a conflict between the two is becoming closer by the hour.   It would be rather amazing that the US, after losing thousands of troops defending South Vietnam, would now support Vietnam if they are attacked by China.   If course history suggests that this is entirely possible as Japan is an excellent example of how former enemies can become close allies.