Thread necromancy ftw!
For the reader, there were warning signs in the episode where Genghis was trapped by Senggum's, Ong Khan's and Jamuka's forces en route to Huazheng's wedding. Jamuka came to negotiate Dukhsh's release, but ended up discussing the dissolution of his andaship with Temujin.
“Brother,” Jamuka said, “You’ve always acted on your own and never
listened to the leaders of the other tribes. Don’t call me ungrateful or
traitor. These past days, you’ve been sending people to convince my
soldiers to join your army by telling them that, once back home, the
resources gained in battle belong to the ones that fought for them. They
won’t be distributed amongst all the tribe’s members. Did you think that
I didn’t know of it?”
“If you are aware of it,” Temujin thought, “then we won’t ever live in
peace with each other.” He then took a small bag out of his clothing and
threw it at the feet of Jamuka. “Here are the gifts that you offered me
when we swore, three times, loyalty to each other. Take them. Later,
when you cut off my head with your saber, you will only kill an enemy
and not a sworn brother. I am a hero, you are also a hero. The plains of
Mongolia are vast, but it cannot contain two heroes.”
While he may have been motivated by a desire to unite the Mongols, in doing so he was prepared to betray his anda, accepting a relationship on his terms only. Later, he told Guo Jing to find and kill Wanyan Honglie.
“Well,” Temujin said. “Since you go to the south, will you bring back
the head of the Sixth Jin Prince, Wanyan Honglie, for me? My sworn
brother Jamuka betrayed me and lost his life, and it’s the fault of that
scum. How many men do you need to achieve this mission?”
After unifying the Mongolian tribes, Genghis Khan posed a serious threat
to the Jin Empire. Confrontation would take place sooner or later, it
was inevitable. Having met Wanyan Honglie on several occasions, Temujin
knew his intelligence and his competence, so it was important that he be
gotten rid of as soon as possible. As for his break with Jamuka, the
true reasons were elsewhere; he had uprooted traditions, had left the
spoils of war to his own warriors, and had sought to attract Jamuka’s
soldiers to his own army. The truth was, they both broke their oath of
fidelity and they did not want to recognize their responsibilities and
preferred to blame it on Wanyan Honglie.