Annam, which is almost the present-day Vietnam, has been reduced to Chinese territory since the beginning of the Qin Dynasty, and it was not until the five generations after the fall of the Tang Dynasty that it began to establish an independent state.
During the Hongwu period of the Ming Dynasty, there was a rebellion within Annam, and its royal family, the Chen clan, died and could not find a suitable heir. Coupled with the fact that cultural customs and other things are basically the same as those in the Central Plains, they asked to join the Ming Dynasty.
At that time, The Ming Dynasty ancestor Zhu Di accepted Annam and changed it to Jiaotong, establishing the Three Divisions and Fifteen Provinces. However, Annan's return was short-lived, and due to some objective reasons, Annam continued to revolt, and it was not until the time of Emperor Xuanzong that he decided to voluntarily abandon his rule over the ann area.
The most direct reason why Emperor Mingxuanzong chose to give up his rule over the Annam region was that he could not afford to raise it. The land of Annam has always been in a state of "not enough land to supply, and not enough to make orders", that is, the Ming Dynasty needed to paste a large amount of silver in order to maintain its rule in the Annam region.
Annam began in the Qin Dynasty and ended after the fall of the Tang Dynasty, and for such a long period of time, although the North belonged to the Central Plains Dynasty and implemented a completely different administrative policy from the Central Plains Dynasty, Annam and the Central Plains Dynasty reached relatively peaceful coexistence.
Then, Annam was separated from the jurisdiction of the Central Plains Dynasty for more than four hundred years, and now you Zhu Di directly implemented the same administrative system as the Central Plains against the people, and once the interests of the local indigenous and powerful people were damaged, how could they not rebel?
If Zhu Di had implemented the toast governance model plus a certain military deterrent, the interests of the local indigenous magnates would have been less damaged, and they might have been a little more peaceful, and the Ming government would not have spent too much silver to suppress it.
And more importantly, an'an's population at that time was as high as three million, more than the population of Henan in the early Ming Dynasty, and if it was to be assimilated, it was necessary to move in a large number of immigrants. At that time, the most neighboring areas of Guangdong, Guangxi, and Yunnan were also very limited in population, and they could not migrate too many immigrants to the past, and they could not be assimilated.
Not only that, but the most critical point is that at that time, Annam already had a certain national consciousness, and the policies implemented by the Ming Dynasty in the local area were too capricious, arousing the local people's indignation. The Ming Dynasty saw that repeated repression had no effect, so it simply gave it up.