Who was Jamuka, and why did he become Genghis Khan’s rival?

Jamuka was a Mongol leader who initially formed a close bond with Temüjin (later known as Genghis Khan) by taking an oath of anda, a pact that made them as if blood brothers.1

The rivalry between the two men emerged from the complex and shifting loyalties of the steppe. While the exact cause of their initial split is obscured by the allusive language of the Secret History, it is clear that their friendship dissolved as both men sought supremacy. Jamuka became a dangerous rival who led coalitions of princes against Temüjin and was even elected gur-khān, or supreme khan, by his followers.2

Their relationship was characterized by polarization; Jamuka was described as an intriguer who was willing to desert allies for short-term gain, whereas Temüjin systematically eliminated rivals to unite the nomadic tribes. Ultimately, Temüjin’s superior leadership and strategic ruthlessness broke Jamuka, leading to Temüjin's consolidation of power and his eventual proclamation as the universal ruler in 1206.2