The man who would become known as Genghis Khan was born as Temujin c. 1162, the son of the leader of the Kiyad clan of Mongols. Legends say that he was born with a blood clot in his fist, a sign that he would be a great leader. He was the third-eldest son in his family, and had two half brothers and a sister. At the age of nine he was sent to the tribe of Borte, with whom his father had arranged a marriage. Temujin was meant to remain with the other tribe until he was 12, the marriageable age. However, as his father returned from leaving his son with the tribe, he was murdered by Tartars, who were enemies of the Kiyad. Temujin returned home to his tribe in the hopes of claiming the leadership, but at nine the tribe was unwilling to allow him to lead them. His mother, Hoelun, his siblings, and himself were all abandoned by the tribe. They spent the next few years in poverty, mainly subsisting on wild fruits and small game hunting. When he was ten he and his half-brother, Bekhter, got into a fight over hunting, and Temujin killed him. In or around 1178 he married Borte, as per his father's wishes. In 1182 he was captured by another Mongol tribe, the Tayichiud, who enslaved him. He escaped shortly after, with the aid of the father of Chilaun, a man who would later be one of Temujin's generals. Around this time his infamy began to grow, and he first became involved with two other men who would later become generals, Jelme and Bo'orchu. In 1184 his wife was kidnapped by Merkits, and he rescued her with the help of Jamuka and Ong Khan.
Temujin began his move into power by offering himself as an ally to Ong Khan, who had been blood brother to Temujin's father. Their relationship was strengthened when Ong Khan helped him rescue his wife, and again when he gave Temujin 20,000 warriors. Though this relationship grew stronger, Temujin's relationship with his childhood friend Jamuka began to grow weaker. In 1190 Temujin was able to unite the Mongol confederation under his leadership, though this confederation was smaller than what it would become. He established the Yassa code, which governed Mongol society. He also went against Mongol tradition by establishing the new confederation as a meritocracy in which loyalty and personal merit were favoured above familial connections. Rather than abandoning conquered tribes, he integrated them into his own, a move which made the conquered peoples loyal to him. In 1201 the final split with Jamuka occurred when Jamuka accepted the title of "universal ruler" and became involved in a coalition against Temujin. Though in 1202 he had been made Ong Khan's heir, his relationship with Ong Khan began to suffer, as Ong Khan's son, Senggum, grew jealous of both Temujin's power, and his closeness with Ong Khan. Ong Khan sided with his son, distancing himself from Temujin. Temujin learned of an alleged assassination plot by Senggum and was able to defeat him. The split between Ong Khan and Temujin deepened when Ong Khan refused to allow Temujin's eldest son to marry his daughter, which was a terrible insult in their culture. Temujin's blood brother Jamuka sided with Ong Khan. Their alliance was wrought with tensions between the two men, and their allies left them for Temujin. In 1203 they were defeated. Jamuka fled to the Naimans, who became the next biggest threat to Temujin's reign. Ong Khan's tribe, the Keirat, on the other hand soon dissolved.
The Naiman Mongols accepted Jamuka into their ranks and refused to surrender to Temujin. However, as had happened with Jamuka and Ong Khan's coalition, many of their allies sided with Temujin. By 1206 they had been defeated in a series of battles, and Jamuka's officers soon surrendered him to Temujin. Temujin had the officers who betrayed Jamuka executed as he did not reward disloyalty, and extended an offer to Jamuka to join Temujin's army. However, Jamuka refused and requested that he be executed by having his back broken, the traditional and honourable method. The remaining Naimans sided with the Merkits, but were defeated by Temujin's generals. This left a majority of the Mongol plain under his control, and all of the other federations were absorbed into his. The Mongol Empire was formed. Also in 1206 the title of Khan was conferred upon him, and he first took the name Genghis Khan.
In 1206 he set his sights on the Tangut's Western Xia Empire, which bordered his own. He began his invasion, well aware that the Jin Dynasty in China would be unwilling to come to the Tangut's aid. Besieging the well-defended cities of the Tangut proved difficult, but was not insurmountable, and he was able to force a surrender by 1209. In 1211 he began his plans to invade the Jin Dynasty. Though tensions between them were mounting, the Jin did not press their advantage and attack first, instead sending a messenger to the Mongols who immediately defected to their side and revealed the location of the Jin Army. This knowledge led to a massacre of the Jin Army by the Mongols. In 1215 he took the Jin capital, Yanjing, which forced the Jin to abandon the Northern half of their kingdom and move south. He next turned west, to the Kara-Khitan Khanate, which had been taken over by one of the Naiman leaders, Kuchlug. By 1218 the Khanate had been defeated and Kuchlug killed. He next turned to the Khwarezmian Empire, an Islamic state located to the West of his extended empire. They began as an ally through which Mongol trade, however Inalchuq, the governor of a Khwarezmian city called Otrar, ordered one such caravan attacked, alleging it contained Mongol spies. He further refused to repay any of the damages incurred by the looting. Genghis sent three ambassadors, two Mongols and one Muslim, to negotiate with the Shah. The Shah responded by having all three men shaved, then decapitated the Muslim, sending the two others back with the head. Insulted, Genghis began to plan a campaign that involved 200,000 soldiers, the largest invasion force he had ever coordinated. He appointed a successor and went to Khwarezmia. There, the Shah had divided his army up into small groups that were as divisive as they were physically divided. The Mongols easily conquered the empire through a combination of superior tactics and strategy. In Otrar, Genghis ordered the slaughter of a large number of citizens, and enslaved those who were not massacred. Inalchuq was executed with molten silver poured into his ears and eyes. The Shah fled before the conquerers, and died mysteriously shortly after. The invasion was exceptionally brutal, even by Mongol standards. By 1222 Khwarezmia was completely under Mongol control. He next moved through Afghanistan and India back towards Mongolia with a majority of his army. Meanwhile, the rest of his army moved through Russia, sacking Georgia and the fortress of Caffa in the Crimea. in 1225 the two factions returned to Mongolia, and ultimately added Transoxiana and Persia to the vast Mongol Empire.
While Genghis had been in the West, the vassal emperor of the Tanguts formed a coalition with the Western Xia and the Jin Dynasty to resist the Mongol rule, banking on the distraction of the Khwarezmia campaign to prevent immediate retaliation. In 1226, immediately after returning to Mongolia, he began a retaliatory attack against the Tanguts. He spent the rest of the year taking a number of Tangut cities, taking the capital in 1227. The Tanguts surrendered soon after, but Genghis ordered the enitre imperial family executed as revenge for the rebellion.
Genghis Khan died in 1227 of unknown causes, though legend holds that he fell from a horse and soon after succumbed to a deadly fever. Others believe it was an illness such as pneumonia, or that he died in battle with the Tanguts. He was 65.