Southeast Asia, called Nanyang in ancient times, is located in the south of China, with a vast territory and rich products. China was a powerful country in ancient times, and Southeast Asia was on China's lips like a piece of fat. However, China never annexed them, only made them vassals, and finally made them cheaper to the western colonists. So why didn't ancient China incorporate Southeast Asia into its territory?

First, ancient Southeast Asia was backward in economy and poor in living conditions. The local area is hot and humid, full of mosquitoes and miasma, and full of virgin forests and swamps, which is not a good place to live. During the Song and Yuan Dynasties, Lingnan area was still desolate and was often used as a place of exile. Southwest China doesn't even have an effective administrative organization, which has always been outside the central dynasty. Nanyang is backward and has poor conditions. Even Lingnan can't compare with it. Of course, the Central Plains Dynasty didn't care. The Central Plains Dynasty believed that Nanyang was an overseas place and did not want to give it away for nothing.

Second, ancient China was an agricultural country and practiced an agricultural economy. Although Nanyang is rich in tropical products, from the agricultural point of view, the local forests are dense, the cultivated land is limited, and the agricultural value is not great, which is far less than the income that makes it a vassal tribute every year. In fact, the great development of Southeast Asia and agriculture is only a recent thing. Almost all places suitable for large-scale farming in East Asia are in China. If Southeast Asia were as rich as the Central Plains, it would have become a part of the Central Plains dynasty. In ancient times, Southeast Asia was of little value to the Central Plains dynasty, so be it.

Third, geographical factors. Although Southeast Asia and China are connected by mountains and rivers, from Vietnam to Myanmar, it is either a deep mountain jungle or a big river swamp. In addition, the local area is hot and humid, and tropical diseases are prevalent. Before the lack of effective antimalarial drugs such as quinine at that time, there were too many non-combat attrition in the army, and China immigrants did not adapt to the local environment, with a high casualty rate. In fact, not only China people are not adapted, but also the indigenous people who have lived there for thousands of years have a high casualty rate. Therefore, although rice is harvested three times a year in Southeast Asia, the population is pitifully small and the degree of development is very low.

Fourth, there were many tribes in ancient Southeast Asia, which were unruly and difficult to tame, and the cost of governance was high. After the formation of nation-states such as Vietnam and Myanmar, it is even more difficult to annex and consume them. Both the Song Dynasty and the Yuan Dynasty tried to recover Vietnam, but both ended in failure. Although the Ming Dynasty conquered Vietnam, it had to give up because of the high cost of occupation and development. The Ming dynasty lost a lot of money in Vietnam, but even if it won Vietnam, the Ming dynasty could not afford to support them. Occupy these places not only can't get much tax, but also have to pay back the money, so no one in the Central Plains Dynasty wants to take over.