Under his nominal rule, the Oirats successfully reunited the Mongol tribes and threatened the Ming dynasty to their south for the first time since the Battle of Kherlen in 1409.
After the previous khan Oyiradai's death in 1425, an internecine war lasting several years broke out among the Oirats under Mahamud (Bahamu, Batula) and western Mongol clans led by Gulichi's family.
Toghtoa Bukha was the eldest son of Ajai who was a posthumous birth of Elbeg Khan (d.1399) and Öljeitü the Beauty.
While wandering with his brothers in western Mongolia, Toghtoa Bukha met Mahamud's son and successor, Toghan Taishi of the Four Oirats.
Toghtoa Bukha Khan was an ambitious leader who aspired to gain real power and regaining the former glory of the Yuan dynasty.
In 1439 Toghan installed Toghtoa Bukha as leader of the eastern Mongols under the title of Bogd Khagan Taisun of the Northern Yuan before the eight white yurts of Genghis Khan.
[citation needed] During the reign of Taisun Khan, Esen Taishi subjugated the Jurchens in Manchuria, the Kara Del kingdom around Hami City, and the Uriankhais (Tuvans) in Siberia.
Esen first directed his attention to the Hami Oasis where the Ming ally, Borjigin prince descended from Chagatai Khan, ruled.
He besieged the city and ravaged its outskirts for 40 days while Esen crushed the Ming armies en route to Beijing.