The Dzungar people had been converted to Tibetan Buddhism in the early 17th century and had a close relation to the Gelugpa church and the Dalai Lamas.
The plan was to subjugate the Khoshut tribe whose pro-Qing ruler Lhabzang Khan had exerted full power over Tibet since 1705.
Another aim was to move the 7th Dalai Lama Kelzang Gyatso to Lhasa; however, the boy was kept safely under Qing control in the Kumbum Monastery, and a Dzungar attempt to take hold of him in 1717 was defeated.
[3] Tagtsepa Lhagyal Rabten was descended from a noble family that resided in the Tagtse Castle to the east of Lhasa, at the right bank of the Kyichu River.
[4] He no doubt belonged to those Tibetan nobles who had a negative view of Lhabzang Khan and, like the monasteries Sera, Drepung and Ganden, entertained contacts with the Dzungar elite.
He appears in the limelight of history in 1717 when Tsewang Rabten's commander Tsering Dondup unexpectedly reached Tibet via the north-western route with 6,000 warriors.
Here Tagtsepa belonged to the traitors who informed the Dzungar troops about the lines of defense of Lhabzang Khan and thereby facilitated the advance of the enemy.
Lhabzang Khan's son Surya and a number of members of the old Tibetan government fled from the Potala after the death of the ruler, seeking refuge in the estate of Tagtsepa in Tagtse.
[7] Tibetan sources confirm that real power rested with the troop commander Tsering Dondup who merely used to new regent as a puppet.
He was manacled and brought to the Chinese camp at the foot of the Potala together with a number of ministers and officials who were accused of cooperation with the Dzungars.