Ong Khan's natural son, Senggum (Senggüm), had been expecting to succeed his father, and plotted to assassinate Temujin. Someone leaked the plans to Temujin. Those loyal to Temujin defeated those loyal to Senggum, and Temujin became ruler of what had been Ong Khan's coalition. In 1206, Temujin the adopted son, took the title Universal Ruler, which translates to Genghis Khan. Genghis (Changez) Khan’s Invasion of Central Asia and Iran
The Nestorian (Persian) Christian influence on the Mongols
The Mongols had living among them a significant number of descendants from the Zoroastrian and Nestorian (Persian) Christian refugees who had fled the Muslim persecution in Persia since the 7th century and had settled in Western China and Mongolia. Among the Nestorian (Persian) Christian refugees many had intermarried with the Mongols and held powerful positions of influence within the Mongol ruling hierarchy. They had also made many Christian converts among the powerful Mongol clans.
In the 13th century, the Mongols finally decided to repel the Muslims who had been making incursions from Kazakhstan into Western Mongolia and China for six hundred years. But in this interregnum of six hundred years, Nestorian Christianity had also made some headway among the Mongol elite at least, certainly those of Kereit clan in origin, most notably the womenfolk in the royal family. The Persian Christian religious identity and activities of Dokuz Khatun, Hulagu’s wife, is documented. Mention can be made of other notable Christian Mongols, such as Kitbugha and Il-Siban, respectively the military commander of Syria in 1260 and the governor (shihna or na’ib) of Damascus who were also Nestorian Mongol Christians.
We need to realize that the ultimate cause for the Mongol attack on Islamdom were the Muslim attacks on Tartary from Persia from 650 upto 1250.