
After the fall of the Yuan dynasty in 1368, the Mongols were pushed back to the Mongolian Plateau, where they established what became known as the Northern Yuan dynasty. This successor state carried on the legacy of the Mongol Empire, albeit fragmented and significantly weakened. The once-mighty empire was reduced to its core Mongolian territories, divided into the Forty Tumens of the Mongols and the Four Tumens of the Oirats, a configuration symbolizing the enduring but diminished strength of the Mongol people.
Early Struggles Against the Ming Dynasty
In 1370, Biligtü Khan Ayushiridara ascended to power as the ruler of the Northern Yuan following the death of the last Yuan emperor. Almost immediately, the fledgling state faced aggression from the newly established Ming dynasty. The Ming sought to assert dominance over the Mongols and launched repeated invasions into Mongolian territory. Despite these efforts, Mongol warlords such as Köke Temür repelled several incursions, notably defeating a Ming force of 150,000 troops at the Orkhon River in 1373. However, the Ming persisted, sacking Karakorum in 1380 and launching further invasions in 1381 and 1392, which were ultimately driven back.
The Ming dynasty also pursued a strategy of weakening the Mongols through diplomacy and economic manipulation. They fomented divisions among Mongol factions and implemented trade embargoes, forcing the Mongols into a state of internal conflict and economic struggle. These policies exacerbated the already volatile state of feudal rivalries among the Mongol tribes.
Decline in Population and Division of the Mongols
By the late 14th century, the Mongol population had diminished due to centuries of war, internal strife, and assimilation into other cultures. The forty tumens, representing the military strength of the Mongols, were reduced to just six, as most Mongol forces had either been lost in China or absorbed into the Ming dynasty. These six tumens were divided into left-wing and right-wing groups, with the left under the direct control of the Mongol khan and the right governed by a vassal known as the Jinong. Meanwhile, the Oirats, who had long maintained their independence in western Mongolia, constituted an additional four tumens.
By this time, Mongolia had become divided into Eastern Mongolia, comprising the Khalkha, Southern Mongols, Buryats, and others, and Western Mongolia, dominated by the Oirats. These divisions contributed to ongoing conflict between eastern and western factions, further weakening the Mongol state.

Location of the Oirats. © Khiruge
Oirat Ascendancy and the Rise of Esen Taishi
The 15th century was marked by the growing influence of the Oirats, who frequently vied with the Eastern Mongols for control. Togoon Taishi, a powerful Oirat leader, played a key role in consolidating power within the Northern Yuan dynasty. His son, Esen Taishi, extended Oirat dominance, temporarily unifying Mongolia under his leadership. Esen's reign culminated in his stunning military victory over the Ming dynasty in 1449. With only 20,000 troops, he defeated a Ming army of 500,000, captured the Zhengtong Emperor, and besieged Beijing.
However, Esen’s ambitious rule was short-lived. He declared himself khan in defiance of Mongol tradition, provoking widespread rebellion among the Eastern Mongols. By 1454, Esen was overthrown and assassinated, leaving Mongolia fractured once again.