Jamukha was resentful of the elevation. Around 1187, the two former blood brothers met in battle at a place called Dalan Baljut, and Temüjin was badly defeated. What followed is one of the most obscure passages of his life. Modern historians believe it likely that Temüjin spent much of the next decade as a servant, possibly even a slave, of the Jurchen Jin dynasty that ruled northern China. A Chinese official named Zhao Hong recorded this explicitly. It was a humiliation so total that every Mongol source omitted it entirely. But Temüjin survived it, and when he re-emerged around 1196, he had lost none of his power to draw followers to his side.

Back on the steppe, Temüjin moved with purpose. In the summer of 1196, he joined the Jin in a campaign against the Tatars, the tribe who had poisoned his father. The Jin rewarded him with an honorific title meaning roughly commander of hundreds. He also helped Toghrul reclaim the Kerait leadership from a usurping relative. These two acts together transformed his standing: no longer Toghrul's vassal in any meaningful sense, he was now a peer. Followers continued to defect from Jamukha, who had treated his own prisoners with extraordinary cruelty after his victory, allegedly boiling seventy of them alive.

Through the early 1200s, Temüjin and Toghrul campaigned together against the remaining rival tribes. Around 1201, a coalition of discontented groups elected Jamukha as their leader and declared war on the Borjigin-Kerait alliance. The coalition was routed. Temüjin then destroyed the Tayichiuds and, in 1202, the Tatars, executing the clan leaders of both and absorbing their warriors into his own forces. Among those absorbed was a young fighter named Jebe, who had killed Temüjin's horse in battle and openly admitted it when questioned. His honesty and courage earned him a place in Temüjin's ranks.

The absorption of the Tatars left three powers on the steppe: the Naimans in the west, Temüjin's Mongols in the east, and the Kereit in between. Temüjin proposed that his son Jochi marry one of Toghrul's daughters, a move the Kerait elite read as a bid for control. Toghrul's son Senggum led the opposition, and Jamukha was there to feed their fears, warning that Temüjin's habit of promoting commoners over nobles threatened the entire traditional order. Toghrul eventually yielded to these pressures and tried to lure Temüjin into an ambush. Two herdsmen overheard the plan and warned him in time. He gathered what forces he could, but was soundly defeated at the Battle of Qalaqaljid Sands.

Retreating to a lake or river called Baljuna, Temüjin waited for his scattered forces to regroup. It was his lowest point since the death of his father. Here, he gathered the men who had stayed with him and swore an oath of loyalty that would become legendary: the Baljuna Covenant. The oath-takers were a remarkable group, men from nine different tribes, Christians, Muslims, and Buddhists among them, united by nothing except loyalty to Temüjin. One historian later called this group a proto-government of a proto-nation. The Baljuna Covenant was subsequently omitted from the official Mongol chronicles because most of its members were not Mongols. That omission tells you everything about what the chronicles chose to celebrate, and everything about what Temüjin actually built.

A feint involving his brother Qasar allowed Temüjin to ambush the Kereit forces at Jej'er Heights. The battle lasted three days but ended in complete victory. Toghrul was forced to flee and was killed by a Naiman who did not recognise him. Senggum escaped to Tibet. Temüjin absorbed the Kereit elite into his own tribe, taking the princess Ibaqa as a wife and arranging marriages between Kereit women and his sons. Then, in May 1204, at the Battle of Chakirmaut in the Altai Mountains, the Naimans were destroyed. Their leader was killed and his son Kuchlug fled west. The Merkits were broken later that year. Jamukha, who had abandoned the Naimans at Chakirmaut, was betrayed by his own companions, who were executed for their disloyalty. According to the Secret History, Jamukha asked his childhood anda to have him killed honourably. Temüjin obliged.

In 1206, at a great assembly called a kurultai held at the source of the Onon River, Temüjin formally took the title Genghis Khan. The exact meaning of the title has never been settled: some scholars read it as meaning universal ruler, connected to the Turkic word for ocean; others see it simply as a statement of supreme authority. Whatever its etymology, its meaning in practice was unambiguous. He was now the sole ruler of the Mongolian steppe, and he intended to remain so.

What followed was a social revolution. Genghis dismantled the old tribal structures and rebuilt Mongol society from the ground up. Every man between fifteen and seventy was conscripted into military units of tens, hundreds, and thousands. Warriors from defeated tribes were scattered across different units to prevent any future rebellion along tribal lines. The highest tier of society was reserved solely for Genghis's own family and his brothers' families, called the Golden Family. Beneath them sat the surviving pre-empire aristocracy and the most important new men. The old tribal identities were to be replaced with loyalty to the Great Mongol State.

His senior companions from the early years were given the greatest commands. Bo'orchu and Muqali, two of his oldest followers, each received ten thousand men. The other companions were given commands of the ninety-five units of a thousand. Many of these men were of low birth: Jelme and Subutai, sons of blacksmiths; a carpenter; a shepherd; even the two herdsmen who had warned Temüjin of Toghrul's ambush in 1203. This was meritocracy by design, and it was radical. The bodyguard corps, the keshig, was expanded from about eleven hundred men to ten thousand. It served simultaneously as the khan's household guard, his military academy, and the centre of government. Entry required being the brother or son of a commander, which made the corps also a system of permanent hostages.

One significant threat emerged from within. The shaman Kokechu, son of Münglig who had been allowed to marry Hö'elün, had proclaimed Temüjin as Genghis Khan and taken the spiritual title meaning Wholly Heavenly. He was enormously influential among ordinary Mongols and began using that influence to divide the imperial family, humiliating Genghis's brothers Qasar and Temüge in public. Börte saw the danger clearly and warned her husband. Genghis, who still superstitiously revered the shaman, allowed his youngest brother Temüge to arrange Kokechu's death. The shaman was killed, and Genghis quietly assumed his spiritual authority as well.

The Western Xia kingdom, ruled by the Tangut people and sitting between Mongolia and the Jin dynasty of northern China, was the first major target. The Mongols had been raiding its borders since 1205. In 1209, Genghis led a full invasion, capturing the fortress of Wulahai, advancing on the Xia capital, and eventually forcing a peace treaty. The Xia emperor Xiangzong submitted, paid tribute, and handed over his daughter Chaka. It was a beginning.

The campaign against the Jin dynasty, which ruled a vast and populous northern China, was a far greater undertaking. Genghis despised the new Jin emperor Wanyan Yongji, and when asked to pay the annual tribute in 1210, he mocked the Jin envoy, spat, and rode away. War followed. In May 1211, he invaded with a three-pronged force, burning and plundering across a huge area of Jin territory, cutting off their supplies and their legitimacy. The campaign was halted in 1212 when Genghis was wounded by an arrow during a siege, but he used the pause to recruit five hundred Jin siege engineers, transforming the Mongol army's ability to take fortified cities.

The campaign resumed in 1213. A detachment under Jebe infiltrated the heavily fortified Juyong Pass and surprised the elite defenders, opening the road to the Jin capital Zhongdu, the city that is now Beijing. The Jin administration collapsed from within: an army commander staged a coup, killed the emperor, and installed a puppet ruler named Xuanzong. The Mongol forces, emboldened and overextended, found themselves camping before Zhongdu's walls while an epidemic and famine ravaged their ranks. Genghis negotiated a peace, securing tribute of three thousand horses, five hundred slaves, a Jin princess, and vast quantities of gold and silk, before withdrawing in May 1214.

Xuanzong then moved the Jin court six hundred kilometres south to Kaifeng. Genghis interpreted this as preparation for renewed war. He returned. On 31 May 1215, the inhabitants of Zhongdu surrendered, and the city was sacked. Genghis returned to Mongolia, leaving his general Muqali in command of the ongoing campaign in China. Muqali waged a brutal and effective war against the weakened Jin regime until his own death in 1223.

By 1218, the Mongol empire bordered the Khwarazmian Empire, a vast state stretching across Central Asia, Persia, and Afghanistan. Trade seemed possible. A caravan of four hundred and fifty merchants, backed by members of Genghis's own Golden Family, set out for Khwarazmia carrying a huge quantity of goods. The governor of the Khwarazmian border town of Otrar massacred them on grounds of espionage and seized their wares. The Khwarazmian ruler Muhammad the Second either approved or looked away. Genghis sent an ambassador to seek redress. Muhammad had the ambassador killed and humiliated his companions. Genghis resolved to destroy him.

The invasion that began in autumn 1219 was the largest operation Genghis had ever mounted. Muhammad's empire was vast but fractured: he ruled alongside his mother in what one historian calls an uneasy diarchy, his troops were garrisoned in cities rather than massed in the field, and his nobility were discontented. Rather than meet the Mongols in open battle, he dispersed his forces behind city walls, which gave the lightly armoured, highly mobile Mongol armies total freedom of movement outside those walls. Otrar fell in February 1220 after a five-month siege. Its governor was executed.

Genghis meanwhile divided his forces into multiple columns. He and his youngest son Tolui took the main army across the Kyzylkum Desert and appeared before Bukhara in a pincer movement. Bukhara's citadel fell in February 1220. Samarkand, Muhammad's residence and one of the greatest cities of the Islamic world, fell the following month. Muhammad fled. Jebe and Subutai pursued him relentlessly until he died of dysentery on a small island in the Caspian Sea in the winter of 1220 to 1221. The two generals then launched what became known as the Great Raid: a circuit of some seven and a half thousand kilometres around the Caspian, bringing the Mongols into contact with Georgia and Kievan Rus for the first time in history.

Muhammad's son Jalal al-Din gathered forces in Afghanistan and won a skirmish against a Mongol column, but Genghis pursued him south and defeated him decisively at the Battle of the Indus in November 1221. Jalal was compelled to swim his horse across the Indus River and escape into India. Meanwhile, Genghis's youngest son Tolui was conducting a separate campaign of annihilation through Khorasan. The great cities of Nishapur, Merv, and Herat, three of the wealthiest urban centres in the medieval world, were destroyed. Persian historians placed the death toll from these three sieges alone at over five and a half million, a figure modern scholars consider vastly exaggerated; even the more conservative estimate of around one and a quarter million dead represents a demographic catastrophe without precedent in the region's history.

Genghis halted the Central Asian campaigns in 1221 and began the long journey home. The return took years. He reorganised the administration of the conquered territories, appointing commissioners and local officials to manage them back to stability. He summoned the Taoist patriarch Changchun to meet him in the Hindu Kush, listened attentively to his teachings, and granted his followers tax exemptions and authority over all monks throughout the empire, a privilege the Taoists later attempted to use to gain superiority over Buddhism. In early 1225, Genghis finally returned to Mongolia.

The Western Xia had refused to provide troops for the 1219 invasion of Khwarazmia, and had grown increasingly defiant while Genghis was away. Diplomacy failed. In the early months of 1226, Genghis launched his final campaign. The Mongols captured Khara-Khoto on the Xia's western border and then proceeded to sack the cities of the Gansu Corridor one by one. By autumn they had crossed the Yellow River, and in November they besieged a city just thirty kilometres south of the Xia capital Zhongxing. On the fourth of December, Genghis decisively defeated a Xia relief army in the field.

But during a winter hunt in 1226 to 1227, Genghis fell from his horse and became increasingly ill in the months that followed. His sons and commanders urged him to abandon the siege and return to Mongolia to recover. He refused. The exact nature of what killed him has never been settled: the sources variously suggest malaria, typhus, bubonic plague, or an arrow wound, while one legend claims the Xia emperor's former wife injured him with a dagger. He died on either the eighteenth or the twenty-fifth of August 1227. His death was kept a closely guarded secret.

Zhongxing, unaware that the khan was dead, fell the following month. The city was put to the sword with extreme savagery. The Xia civilisation was, in the words of one historian, essentially extinguished. Genghis was carried back to Mongolia and buried near the sacred Burkhan Khaldun peak in the Khentii Mountains, on a site he had chosen years before. The mountain was declared a prohibited zone, guarded by his Uriankhai followers. The precise location of his grave has never been found.

The succession he had arranged before the final campaign held. His third son Ögedei, chosen over his militarily superior younger brother Tolui because of his generosity, his judgement, and his ability to compromise, was crowned khan at a great assembly in 1229, after a two-year regency. Jochi, the eldest son whose parentage had always been questioned, had predeceased his father by a matter of months, dying of illness in 1227. Chagatai, the second son, was judged too arrogant and inflexible. Tolui, though the greatest commander of his generation, was passed over in favour of a man who could hold the empire together rather than simply conquer more of it. It was, historians agree, a sound judgement.

Genghis Khan left behind the largest contiguous empire in world history, stretching from the Pacific coast of China to the edges of Eastern Europe. He had unified the Mongol tribes, destroyed the Western Xia, broken the Jin dynasty of northern China, annihilated the Khwarazmian Empire, and sent his generals raiding as far as Georgia and the Russian steppe. The empire he founded would eventually fracture into successor states: the Golden Horde in Russia, the Chagatai Khanate in Central Asia, the Ilkhanate in Persia, and the Yuan dynasty in China. But for a generation, the Silk Road ran under a single authority, and the commercial and cultural exchange it enabled was without precedent.

His legacy is genuinely double-edged, and no honest account can resolve the contradiction. The prosperous civilisations of Central Asia and Persia suffered multi-generational trauma from the Mongol campaigns. In the Arab world and in Russia, Genghis is remembered as a savage destroyer, the man whose grandson Hulegu sacked Baghdad in 1258 and whose Golden Horde imposed what Russian historians call the Tatar Yoke on their country for two and a half centuries. These are not misreadings of history; they are what the history was.

And yet the empire he built was also genuinely meritocratic, legally sophisticated, and religiously tolerant in ways that were remarkable for its era. He appropriated the Uyghur alphabet to give the Mongols a written language. He established an independent judiciary. He promoted men by ability regardless of birth, and his bodyguard corps included Christians, Muslims, and Buddhists. The Baljuna Covenant, sworn at his lowest point with men from nine different tribes, became the model for the empire that followed. The Washington Post, in the mid-1990s, named him man of the millennium, describing him as someone who embodied the half-civilised, half-savage duality of the human race. That formulation, however uncomfortable, is not wrong.

In Mongolia itself, Genghis was remembered for centuries primarily as a religious figure rather than a political one. After the conversion of the Mongol nobility to Tibetan Buddhism in the late sixteenth century, he was formally deified, connected to the Buddha, invoked at weddings and festivals, and made the centre of a sleeping hero legend, which held that he would return to help his people in a time of great need. His cult was suppressed under Soviet influence in the twentieth century, the alphabet he chose replaced with Cyrillic, and celebrations planned for the eight hundredth anniversary of his birth in 1962 were cancelled under Soviet pressure. Then, after the 1990 revolution, the rehabilitation was swift: Lenin Avenue in Ulaanbaatar was renamed Chinggis Khan Avenue, the international airport was given his name, and his face appeared on banknotes and postage stamps. The Mongolian parliament in 2006 debated whether his name had been trivialised by appearing on brands of alcohol and toilet paper.

What remains, beneath the deification and the demonisation, is the outline of a man shaped entirely by the world that nearly destroyed him in childhood. He valued loyalty above every other virtue because he had known what it was to have none. He promoted men by merit because the tribal system that rewarded birth had abandoned his family on the steppe. He built an empire on the oath sworn at Baljuna, among men from nine different tribes who shared nothing except the fact that they had stayed. The blood clot he was said to have clutched at birth was a legend. The grip he kept on every follower who ever pledged to him was not.