The King was called Genghis Khan,
Who in his time was of great renown
That there was nowhere in no region
- The Squire's Tale
Chapter 3: The Unification
Around the time of Temuujin, the Central Asian Plateau was divided into several tribes including Naimans, Merkits, Tatars, Khamag Mongols, and Keraits. These tribes randomly raided each other. Temuujin went on to unite all these tribes into one single unit.
Rise to Power
He began his rise to power when he offered himself as an ally to his father's anda (sworn blood brother) Wang Khan, otherwise known as Toghrul. Temuujin's father-in-law had given him a coat as his wedding present. He offered this coat to Wang Khan as a present. Their relationship was further strengthened when Borte was kidnapped by the Merkits. Temuujin turned to Wang Khan for help in getting her back. Together with Wang Khan, and his childhood anda, Jamukha, Temuujin was successful in getting her back and defeating the Merkits. This would mark the beginning of a long feud between Jamukha and Temuujin over which one would become the bigger leader. Temuujin would prevail.
Power struggles
But first, relationships between Temuujin and Wang Khan weakened. First, Wang Khan's son, Senggum grew jealous of Temuujin's power. Convinced by his son, Wang Khan started to become uncooperative with Temuujin. After Temuujin found out about Senggum's intentions, he defeated him and his followers. Wang Khan later refused to give his daughter in marriage to Jochi, Temuujin's eldest son. This was a sign of disrespect and served as a pretext for war. Wang Khan allied himself with Jamukha. However, because of conflict between the two, Wang Khan lost to Temuujin, and Jamukha escaped during the battle, only for his few remaining followers to betray him and bring him back to Temuujin. With Jamukha in front of him, Temuujin offered Jamukha the option to become andas again. Jamukha refused, saying that it would benefit Temuujin if he died. Temuujin ordered his followers to kill Jamukha in a respectable manner: by breaking his spine (this did not spill any blood).
As for Jamukha's followers (who betrayed him), Temuujin ordered them to be killed. He respected loyalty more than all else.
Rival tribes and their defeat
The rival tribes of the Mongolian confederation included the Naimans to the west, the Merkits to the north, Tanguts to the south, and the Jin and Tatars to the east. In order to unite all these tribes, Temuujin broke away from traditions. He gave people authority based on how loyal and skilled they were, not by family ties. Also, he gave the conquered people equal rights as his own people. In other words, he did not make them slaves.
After constant warfare with his neighboring tribes, he was able to unite them into a single unit, and thus, in 1206, became the sole ruler of the Mongol plains. It was then that he became Chinggis Khaan.
For him, uniting the steppe tribes was not enough. He went on to conquer much more.