Vietnam army ! be careful ! Chinese has a traditional habit : word and act are not the same. Their ex-leader has taught them " Negotiation is negotiation, attack is acttack "...and Sino army applied in 1979.
Year 1978, Vietnam leader - Le Duan came to China for a "friendship" negotiation. February of 1979 ( just few months after), suddenly we got Sino war. Vietnam leaders believed in Chinese peaceful word so much that the Viet army ( at that time was very strong and seasoned ) got a big surprise and damage very much. After 5 days, they found out what was happening, the central army woke up ( at that time they just worry about US army, considering China as good friend ), order to fight back and push Sino solider out of the Viet land.