Vietnam has intermittently been under the jurisdiction of the Central Plains dynasty in history. The last time it was under such jurisdiction was during the Ming Dynasty, but it regained independence during the Qing Dynasty.

Essentially, this is due to Vietnam's geography and topography:

The northern part of Vietnam is separated from Guangxi in the Central Plains by mountains, making it difficult to enter and exit. The second reason is more important: Vietnam is a long and narrow country, with a distance of up to 1600km from north to south. Its contact with the Central Plains is at one end. This kind of terrain makes it easier to conquer the northern end, but as you move south, it becomes increasingly difficult for your ruling power to penetrate. Therefore, when the Central Plains dynasty is not strong enough, it is easy for the southern end to gather enough strength to overthrow the Central Plains officials in the north, or for the northern officials to simply declare themselves as kings, as was the case with Zhao Tuo during the Qin Dynasty.

Another place with similar geographical conditions to Vietnam is the Hetao Plain. The Hetao Plain is a long and narrow plain formed by the Yellow River turning in Ningxia and Inner Mongolia, with Shanxi on the right and some deserts between Zhongwei City and Lanzhou on the far left. For the Central Plains dynasty to attack the Hetao region, they would have to attack from both ends, but due to the narrow terrain, it is not easy to launch an attack. If they were to cross the Loess Plateau and directly attack the middle, the difficult terrain of deserts and gobi would make it hard to supply troops. Therefore, historically, the Song Dynasty and the Western Xia were able to hold sway for over a hundred years.

This kind of terrain is referred to as "distant" in Sun Tzu's Art of War, which is not conducive to attack.

On a side note, I have been on duty in Zhongwei a few times, and one time I had a nosebleed from eating too many goji berries 😂

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