"That's a pretty hard thing to explain." Well, the vietnamese army say that they did it by leaving 10 000 vietcong in the south after the separation and rapidly subsumed all resistance movements as front organisations under direct northern control. The south was thoroughly corrupt, brutal and inept but having another state sponsoring and controlling a guerilla war in your country could not have helped. https://www.amazon.com/Victory-Vietnam-Official-History-1954...