This is mainly because Mongolia is a nomadic people, and the main purpose of Mongolia's expansion is to obtain high-quality pastures. From the Mongolian grassland to the east and west, there are grasslands all the way, and you can graze cattle and sheep. Vietnam, Thailand and other places in the south, with dense forests and complex terrain, are not convenient for Mongolian cavalry to fight. After Mongolia seized these territories, it was of little significance to the Mongols.

Mongolia is a grassland tribe that rose quietly in the background of the dispute between Song and Jin in the late Southern Song Dynasty. Under Temujin's leadership, Mongolian cavalry was eliminated successively, including Jin, Xixia and Southern Song Dynasty. The Mongolian people's desire for territory is almost endless, so under the leadership of Temujin, Mongolian cavalry continues to expand, and to the east, it is Japan across the sea. Unfortunately, Mongolia's eastward expedition to Japan was wiped out by two consecutive hurricanes.

Mongolia's western expedition is invincible all the way. It has successively wiped out dozens of countries and expanded the territory of the Mongolian empire to more than 50 million square kilometers, making it the largest empire in human history. After the Southern Song Dynasty, Mongolia did not continue to go south to attack Thailand, Myanmar and Vietnam at that time. The main reason is that the farther south, the denser the water network and jungle, and the horses of Mongolian cavalry can hardly enter such areas, and their high mobility can hardly be exerted. At that time, Vietnam, Myanmar and Thailand were all mainly agricultural countries, and this Mongolian nomadic tradition was completely different.

On the contrary, the ancient Han Dynasty expanded to Vietnam and Thailand many times, mainly because China was an agricultural country like these countries at that time, and seizing their territory could expand agricultural planting area and enhance national strength.