Who was Genghis Khan's childhood friend and later rival?

Genghis Khan’s childhood friend and later rival was Jamukha. The two boys formed an anda relationship, a sworn-brother bond that carried strong obligations in Mongol society. Their friendship is described in The Secret History of the Mongols, one of the principal sources for Temüjin’s early life.

For a time, Jamukha and Temüjin shared followers and cooperated militarily. Jamukha also helped Temüjin recover Börte after the Merkits kidnapped her. Their political ambitions eventually pulled them apart. Temüjin built a broader coalition by rewarding loyalty and incorporating people from different tribes, while Jamukha represented a more aristocratic style of steppe leadership.

Their rivalry became a struggle for supremacy among the Mongol tribes. In 1201, opponents of Temüjin proclaimed Jamukha Gur Khan, or “Universal Ruler.” Temüjin defeated the coalition, was proclaimed Genghis Khan in 1206, and ultimately prevailed. Jamukha was later handed over to Temüjin, who ordered his execution without spilling his blood, according to traditional accounts.