This article has a total of 1989 words and a reading time of about 4 minutesBefore we ask this question, we need to be clear that ancient China did not have a clear concept of territory like modern countries. The main purpose of the Southern and Northern Expeditions was to ensure their own security, rather than to pursue territorial expansion, and it was not until the Qing Dynasty that the concept of territory gradually developed.
In ancient Chinese history, both the Ming and Qing dynasties expanded their territory to the west, while the expansion to the south was relatively small. This raises the question of why the Ming and Qing dynasties chose to expand to the west rather than to the south (Vietnam, Thailand, Burma, and Vietnam).
Let's talk about why the Ming and Qing dynasties didn't expand to the south
It was true that the Ming Dynasty had been expanding southward, but due to the distance between the northern and southern fronts, the Ming still faced intrusion from the Mongol tribes in the north after eliminating the Yuan Dynasty. Therefore, after Zhu Di ascended the throne, the whole country moved north, and established the principle that the Son of Heaven guards the country and the king dies.
However, the small princes of the south understood the general trend, and once they saw that the Ming Dynasty army was strong, all the countries of Southeast Asia declared themselves subordinate to the Ming Dynasty and said that they would pay tribute every year. In order to facilitate the reasonable collection of taxes, the Ming Dynasty set up three Xuanfu Divisions and six Xuanfu Divisions, of which the southernmost Sumatra Old Port Xuanfu Division was located in the old port city of Sumatra, present-day Indonesia. 
The Xuanwei Division was ruled by the ** stationed in the Central Plains, and was later succeeded by a hereditary system. Although it is nominally a small ** of the Ming Dynasty, it is actually equivalent to the local emperor.
In the last years of the Ming Dynasty, due to corruption and national chaos, all parts of Southeast Asia became independent and established their own small states. During the Qing Dynasty, as the ruler of the Central Plains by foreign tribes, its military power was relatively weak. After the establishment of the Qing Dynasty, it mainly relied on the Han Chinese army to suppress domestic uprisings. When the Qing Dynasty was first established, its rule was mainly based on the Manchu and Mongolian ethnic groups, so the Qing Dynasty focused most of its efforts on unifying the Mongol ethnic groups. 
The Qing Dynasty often devoted itself to pacifying the spheres of influence formed by the various ethnic groups of the Mongols and achieving domination over the Mongol ethnic regions, and these wars consumed a great deal of the Qing Dynasty's national strength. Later, the Qing Dynasty also tried to expand into the southeast, and after the Great and Small Jinchuan Rebellion, the Qing Dynasty realized that the people in the southeast were strong and the victory was costly.
If the Qing Dynasty insisted on expanding to the southeast, it could create a risk of collapse for the entire Qing Dynasty. Coupled with the fact that the Qing Dynasty already ruled a fairly large territory at that time, there was no need to expand further south. 
Although the Ming and Qing dynasties were in different situations, they all belonged to a part of the ancient Chinese civilization, and then analyzed the development characteristics of the ancient Chinese civilization, which was mainly reflected in the following aspects:
In the northwest of ancient China, there were often barbaric nomadic peoples, such as Warat and Jungar, who had always harbored ambitions and constantly attacked the Central Plains Dynasty. These nomads were skilled horseback riders, and in winter, their pastures were barren due to lack of forage, and the southern meadows had just finished their autumn harvest and became the target of their plunder.
In particular, the more militarily powerful minority regimes of the Dzungar Ministry have always hoped to reach an alliance with the **lama and **tsar, unify the whole of Mongolia, and then occupy the whole of China, and reproduce the glory of the Genghis Khan period. 
Therefore, they stationed heavy troops on the border and frequently attacked border cities. Under these circumstances, the Ming and Qing dynasties could only passively send troops to attack these minority regimes in the northwest, so as to expand to the northwest, and the essence was to maintain national peace.
In contrast, Southeast Asia has relatively few problems. The people there rarely showed a desire to expand northward, especially under the Ming dynasty, and since its establishment in Yunnan, the small states of Southeast Asia have made claims to the Ming and are willing to pay annual tribute.
The Central Plains Dynasty has always been dominated by cavalry units with strong mobility and excellent combat effectiveness. Southeast Asia is a region with many terrains, high mountains and lush forests, which are not suitable for large-scale cavalry marches. Fighting in the plains of Southeast Asia is not only difficult to give full play to the superiority of cavalry, but also relatively risky. 
In contrast, the western and northwestern regions are open and vast, and the cavalry units can not only roam freely, but also the western part is high and has many important passes. The occupation of the western region could create a natural barrier for the Central Plains Dynasty.
The northwest region of our country is dominated by plains, with flat terrain and sparsely populated, which is more suitable for military conquest. For a long time, the minority regimes in the northwest had become Xi to fighting with the Central Plains Dynasty and had become familiar with the routes of their troops. Therefore, in terms of geographical conditions, the north-west is more suitable for large-scale wars.
In the South, however, the situation is quite different. The area bordering southern countries such as Vietnam and Myanmar is a vast tropical rainforest with a difficult terrain that is not suitable for large-scale military operations. In addition, the presence of the Himalayas also forms a natural barrier, making it extremely difficult to attack foreign countries. 
Most of China's ancient regimes had a common characteristic, that is, they focused on safeguarding territorial integrity and did not take the initiative to expand externally. As a result, most of the military operations are passive. Compared with countries such as Vietnam and Burma in the south, the Central Plains regime has always maintained peace with them and has not had large-scale wars. Although there were local small-scale wars with Burma during the Qianlong period, on the whole, relations between the Central Plains regime and the southern countries were relatively harmonious.
The Northwest region is rich in natural resources and is also at the intersection of important trade routes such as the Silk Road, which can greatly support the development of the Central Plains Dynasty and consolidate its dominance. 
Since the opening of the Silk Road in the Han Dynasty, the western region has become an important overland trade route between China's hinterland and Europe. The Silk Road was a major channel for economic, political and cultural exchanges between the East and the West, and was of great economic importance to the imperial court and could not be ignored.
The small countries in the south have relatively few products, relatively poor economies, and close proximity to the sea, and there are relatively few connections between countries, and the benefits to the Central Plains Dynasty are not very significant. To sum up, there are many reasons why the Ming and Qing dynasties chose to expand to the west rather than the south (Vietnam, Thailand, Burma, and Vietnam). Economic, political, and topographical factors all had an important impact on the direction of expansion of the Ming and Qing dynasties.
The strategic choices of the Ming and Qing dynasties were closely related to the national interests and practical conditions at that time. For Chinese history, this is a historical period worthy of in-depth study and exploration.