The military machine Genghis built was extraordinary. It rested on the Mongol horseman, the finest light cavalry in the world, trained from childhood to ride and shoot simultaneously. Each warrior travelled with multiple horses, switching mounts to maintain speed over vast distances. The army functioned without fixed supply lines, foraging as it moved. Discipline was instilled through the nerge, a traditional hunt in which thousands of riders would spread out across an entire region, drive all game inward, and slaughter everything, permitting nothing to escape. As a training exercise it was also a rehearsal for encirclement warfare. And the Mongols added to their cavalry the siege engineers, trebuchet operators, and technical specialists they captured from the empires they conquered, turning nomadic speed into a force that could crack city walls.

The campaigns that followed were staggering in their scope. Genghis moved first against the Jin dynasty of the Jurchens in northern China, then against the Western Xia of the Tanguts. He moved west against the Khwarazmian Empire, a vast Persian-speaking realm that stretched across Central Asia, after its shah made the catastrophic mistake of massacring a Mongol trade delegation. The response was total. City after city was taken, and those that resisted were destroyed. By the time Genghis died on 18 August 1227, the empire he had built stretched from the Pacific Ocean to the Caspian Sea. It was twice the size of the Roman Empire at its height. It was the largest contiguous state in recorded history, and it had been assembled in about twenty years.

Genghis was buried in a secret location, as Mongol tradition required. He had already named his heir: his third son Ögedei, described as charismatic and capable, the one son the various factions could accept. A regency held by Ögedei's younger brother Tolui kept the empire together until the formal kurultai of 1229 elected Ögedei as great khan. The transition worked. The empire did not fracture. It expanded.

Under Ögedei the conquests accelerated in every direction. In China, his general Subutai captured the Jin capital of Kaifeng in 1232 after a major siege, and the Jin dynasty finally collapsed in 1234. In the west, his general Chormaqan destroyed the last remnant of the Khwarazmian Empire. In Korea, the king of Goryeo surrendered, then revolted and moved his court to an island fortress, beginning a long, unresolved occupation. The Mongols established their capital at Karakorum in 1235, building a palace city on the steppe that would become the administrative heart of the empire for the next quarter century.

The most dramatic campaign of Ögedei's reign was the push into Europe. A grandson of Genghis named Batu Khan led the western army, with the veteran general Subutai as his operational commander. By 1237 they were at the borders of the Kievan Rus principalities, the network of city-states that covered modern Russia and Ukraine. Ryazan was the first to fall, taken after three days of fierce fighting, its population massacred. The army of the Grand Principality of Vladimir was destroyed at the Battle of the Sit River. By 1240, virtually all of Kievan Rus had fallen. Kiev itself was reduced to ruins.

The advance continued west into Poland and Hungary. A European coalition of Poles, Moravians, and military orders including the Hospitallers and the Teutonic Knights tried to stop the northern flank and slowed it briefly at Legnica. In Hungary, the army of King Béla the Fourth and his allies was crushed at the Battle of Mohi on 11 April 1241. In a single night during that campaign, thirty thousand Mongol cavalry crossed the flooded Sajó River, a feat of military engineering that stunned the defenders. The road to Vienna was open. Then Ögedei died in December 1241, and everything stopped.

By Mongol law and custom, every prince of Genghis's bloodline had to return for the kurultai to elect a new great khan. Batu withdrew his forces from Central Europe. Researchers today doubt Ögedei's death was the only reason; the fortified castles of Central Europe, the lengthening supply lines, and difficult terrain all played a part. But the invasion ended. Western Europe never faced the Mongols again. The empire's western limit in Europe had been drawn, not by any army that defeated it, but by a death in a palace on the other side of the world.

The years between Ögedei's death in 1241 and the election of the next great khan were years of dangerous uncertainty. Ögedei's widow Töregene seized the regency and held it for more than four years, building palaces and cathedrals, supporting religion and education on an imperial scale, and manoeuvring to secure the throne for her son Güyük. Batu, ruler of the western Golden Horde, refused to attend the kurultai, claiming illness, his absence a calculated act of defiance that stalled the process. The empire held together, but the cracks were widening.

Güyük was finally elected in 1246, his campaigns in Manchuria and Europe giving him enough prestige to win the vote despite his deteriorating health. He was ill and heavily dependent on alcohol by the time he took power. He moved quickly to dismantle his mother's legacy, punishing her supporters and restoring his father's officials. He continued military operations in Korea, advanced against the Song dynasty in the south, and ordered an empire-wide census. Then, in 1248, he raised troops and marched westward from Karakorum. His stated reason was unclear. Some said he was heading to his personal estate to recuperate. Others believed he was moving to launch a full invasion of the Middle East. Others still suspected he was marching to confront Batu Khan directly.

Sorghaghtani Beki, the widow of Genghis's son Tolui and one of the shrewdest political minds in the empire, secretly warned Batu that Güyük was coming. Batu was already travelling eastward. The two armies never met. Güyük, sick and exhausted by travel, died en route in Xinjiang, possibly poisoned. His widow attempted to hold the regency, but she lacked her mother-in-law's political skill, and her authority collapsed quickly. Batu called his own kurultai and nominated Möngke, a son of Tolui, as great khan. The Ögedeid and Chagataid families refused to attend, insisting only Ögedei's descendants could hold the title. The impasse was broken when Möngke's mother Sorghaghtani organised a second, more formal kurultai on 1 July 1251. The Toluids had won.

Möngke's accession was a turning point. Power passed from the line of Ögedei to the line of Tolui, and Möngke enforced the transition with a purge. Investigations were launched across the empire. Between seventy-seven and three hundred members of the Mongol elite were found guilty of plotting against him and executed, though princes of the royal bloodline were usually exiled rather than killed. Möngke confiscated the estates of the Ögedeid and Chagataid families and shared the western empire with his ally Batu. The Toluid dynasty was now in control, and it would remain so.

Möngke proved to be a serious and capable ruler. He was tolerant of all religions, and the capital Karakorum under his reign became a cosmopolitan city where Buddhist monasteries, mosques, and Christian churches stood alongside each other. The most famous decoration in the city was a great silver tree, crafted by a Parisian goldsmith named Guillaume Boucher, with cleverly designed pipes that dispensed different drinks from its branches, topped by a triumphant angel. Möngke launched sweeping economic reforms, fixing government spending, commuting the old contribution system to a predictable poll tax, and reducing the burden on commoners. He ordered an empire-wide census in 1252 that took years to complete, reaching as far as Novgorod in 1258.

He assigned his brothers to govern the empire's two most contested frontiers: Hulagu to Persia, and Kublai to China. In China, Kublai conquered the Dali Kingdom in Yunnan in 1253, opening a southern route to outflank the Song dynasty. In Vietnam, a Mongol army pushed into the kingdom of Đại Việt in 1258 but was forced to withdraw. Möngke himself led the main army south against the Song, but the campaign ground into a prolonged and costly struggle. Disease ravaged the Mongol forces. Möngke Khan died on 11 August 1259, on campaign in southern China. The empire he left behind was the largest single political entity the world had ever seen. It would never again be governed as one.

Möngke's death triggered the crisis that had been building for a generation. His brother Hulagu, in the middle of his devastating campaign through the Islamic world, broke off his advance into Syria and withdrew the bulk of his forces east, leaving only a small contingent behind. That contingent was destroyed by the Egyptian Mamluk army at the Battle of Ain Jalut in 1260, in a valley just north of Galilee. The Mongols were defeated. It was the first time a Mongol army had been beaten in open field in the Middle East, and it marked the permanent western limit of Mongol expansion. They never advanced beyond Syria again.

The succession war was already underway. Möngke's younger brother Ariq Böke used his position at the capital of Karakorum to have himself proclaimed great khan by a kurultai there. Simultaneously, another brother, Kublai Khan, who had been campaigning in China, summoned his own kurultai at Kaiping and was proclaimed great khan by the princes and commanders of northern China and Manchuria. Two great khans. Two armies. The Toluid Civil War had begun.

Kublai's forces proved stronger. He blockaded Ariq Böke in Mongolia, cutting off food supplies and causing a famine. Karakorum fell to Kublai, then was retaken by Ariq Böke, then fell again. In the southwest, Hulagu backed Kublai, but his cousin Berke, the Muslim ruler of the Golden Horde, backed Ariq Böke and clashed with Hulagu in a series of conflicts that began in 1262. Hulagu died on 8 February 1264. Berke moved to invade but died along the way. Ariq Böke surrendered to Kublai at Shangdu on 21 August 1264. The war was over, but the unity of the empire was not restored. It was finished.

Kublai Khan was now the nominal great khan of the Mongol Empire, but his real power was concentrated in the east. In 1271 he formally renamed his Chinese domain the Yuan dynasty and moved his headquarters to Khanbaliq, the city that would later become Beijing. Many Mongols accused him of being too closely tied to Chinese culture, too willing to adopt the sedentary, bureaucratic ways of the civilisations he had conquered. They were not entirely wrong. Kublai was building something new: not a nomadic empire but a Chinese dynasty with Mongol rulers, complete with an academy, trade ports, canals, public schools, and an extensive postal relay system of some fourteen hundred stations.

The final conquest of the Song dynasty in southern China came in 1276, when the Chinese imperial family surrendered to the Yuan. The Mongols became the first non-Chinese people to rule all of China. Kublai pushed further: his armies invaded Burma and Sakhalin, and he launched two separate seaborne invasions of Japan, in 1274 and again in 1281. Both failed. In Vietnam, his armies were beaten repeatedly and finally crushed at the Battle of Bạch Đằng. The limits of Mongol power were becoming clearer with each campaign. On land, across the steppe and through settled cities, the system worked. On water, in jungles, in the rice paddies of Southeast Asia, it did not.

The formal structure of the empire during these years was a fiction of unity covering four sovereign states. The Golden Horde controlled the northwestern steppe and held Russia in tributary submission. The Chagatai Khanate ruled Central Asia. The Ilkhanate governed Persia and Iraq. The Yuan dynasty ruled China and nominally claimed supremacy over the others. In 1304, during the reign of Kublai's grandson Temür Khan, the three western khanates formally accepted Yuan suzerainty in a peace treaty. It was the last moment anything resembling a unified Mongol order existed. Each khanate was already going its own way.

For all its violence, the empire in its maturity created something historians call the Pax Mongolica, the Mongol Peace: a period of relative stability across Eurasia during which trade, technology, and ideas moved with unprecedented freedom along the routes that connected China to the Mediterranean. Merchants from Italian cities formed trade partnerships with Mongol elites. The explorer Marco Polo travelled east along what became known as the Silk Road. A Nestorian monk named Rabban Bar Sauma made the reverse journey, travelling from the Mongol capital all the way to Europe. European missionaries visited the Mongol court. Ambassadors carried letters between popes and khans.

The postal relay system, known as the Yam, made all of this possible. Relay stations were set up across the entire empire, spaced roughly forty kilometres apart, each with fresh horses and provisions. Mongol riders regularly covered two hundred kilometres in a single day, faster than any comparable system for centuries. When the great khan died in Karakorum, news reached Batu Khan's forces in Central Europe within four to six weeks. Patterns of Yuan royal textiles appeared on Armenian decorations. Trees and vegetables were transplanted across the empire. The astronomer Tusi, freed from Baghdad, built the Maragheh Observatory in Persia in 1259 under Hulagu's patronage, saving some four hundred thousand books from the siege of Baghdad, and his innovations in mathematics would eventually influence Copernican astronomy in Europe.

The Silk Road also carried something else. The plague known as the Black Death almost certainly travelled west along the Mongol trade routes. In 1347, Mongol forces besieging the Genoese trading post of Caffa on the Crimean Peninsula, their own army already dying of disease, catapulted infected corpses over the city walls. The Genoese fled by ship to southern Europe, carrying the plague with them. The total death toll worldwide is estimated at between seventy-five and two hundred million people, with perhaps fifty million in Europe alone. The Mongol Empire's greatest gift to the world and its deadliest export travelled the same roads.

The death of the Ilkhan Abu Said in 1335 began the final dissolution. His successor was murdered within a year. Persia fell into political anarchy, divided between warlords and rival governors. The Georgians pushed the Mongols out of their territory. An independent state emerged in Anatolia. The loyal Armenian Kingdom of Cilicia, stripped of Mongol protection, was overrun by the Mamluks in 1375. The Ilkhanate, the southwestern khanate that had sacked Baghdad and reached the edge of Egypt, simply ceased to exist as a coherent state between 1335 and 1353.

In China, the Yuan dynasty entered its own spiral of civil war and misrule. The last Yuan ruler, Toghan Temür, was powerless to control the chaos around him. His court's paper currency entered a hyperinflationary collapse. The Han Chinese population revolted against decades of harsh Mongol impositions. In Korea, the king Gongmin successfully expelled Mongol garrisons in the 1350s and exterminated the family of Toghan Temür's empress. In Tibet, a local leader named Tai Situ Changchub Gyaltsen eliminated Mongol influence entirely. The empire that had once claimed to rule the world was losing control of its own provinces.

The Ming forces, a Han Chinese rebellion that had grown into a dynasty, advanced on Khanbaliq. In 1368, Toghan Temür fled north to the Mongolian plateau. The Yuan dynasty was over. The Mongols had ruled all of China for less than a century. They retreated to the steppe from which they had come, and the rump state that remained on the Mongolian plateau, known as the Northern Yuan dynasty, would endure in diminished form until the Qing dynasty conquered it in the 1630s.

The Golden Horde followed its own slower decline. It had controlled the Russian principalities for over a century, collecting tribute and keeping the princes in submission. Moscow had actually risen to prominence during this period, its rulers trusted by the Mongols as tax collectors. But the Horde fractured into competing successor khanates through the late fourteenth and fifteenth centuries. The decisive moment came in 1480, at the Great Stand on the Ugra River, where Ivan the Third, grand prince of Moscow, faced the Horde's forces across the water and neither side crossed. The Horde withdrew. Ivan declared independence. The Mongol grip on Russia was broken.

The Chagatai Khanate in Central Asia lasted longer, but in fragmented form. The two main successor states were destroyed by Timur, the conqueror known in the West as Tamerlane, who built his own Timurid Empire from their ruins in the late fourteenth century. A remnant called the Yarkent Khanate survived until 1705, when it was defeated by the Oirat Dzungar people. The Crimean Khanate, a successor of the Golden Horde, endured until 1783. Other successor states, including the Khanate of Bukhara and the Kazakh Khanate, lasted even longer. The empire did not end on a single day. It dissolved, piece by piece, over four centuries.

The Mongol Empire killed on a scale that is almost impossible to comprehend. Estimates of the total death toll from the conquests range from thirty million to eighty million people. China's population may have fallen by half during fifty years of Mongol rule, from around one hundred and twenty million to roughly sixty million. Iran's population may have lost three-quarters of its people to warfare, famine, and disease, and historians estimate it did not return to pre-Mongol levels until the middle of the twentieth century. About half of Hungary's population may have died in the invasion of 1241. The destruction of Baghdad in 1258 ended five centuries of Abbasid civilisation and dispersed the accumulated knowledge of the Islamic world.

And yet the same empire built the most sophisticated postal relay system the medieval world had ever seen, faster than the American Pony Express six hundred years later. It created the conditions for the Silk Road to function at its fullest extent. It spread astronomical knowledge from Persia to Europe. It produced the oldest surviving literary work in the Mongolian language, a record of Genghis Khan's life written for the royal family after his death. Its Mongol alphabet, adapted from the Old Uyghur script, is still used in Mongolia today. The empire that destroyed Baghdad also saved four hundred thousand of its books.

The through-line of the Mongol Empire, from the abandoned child on the steppe to the fractured khanates of the fifteenth century, is the question of what happens when a people with no cities, no fixed borders, and no written tradition of governance suddenly finds itself responsible for half the known world. The answer the Mongols gave was remarkable and terrible in equal measure: they absorbed the bureaucrats and astronomers and engineers of the civilisations they conquered, granted religious freedom to everyone, built roads and relay stations across five thousand kilometres of steppe and mountain and desert, and held it all together through the force of a legal code and a military system that had no precedent. When the system finally broke, it broke not from outside pressure but from within, from the same question that had haunted it since Börte's first son was born with uncertain parentage: who, among Genghis's descendants, had the right to rule what he had made.

The world Genghis Khan built outlasted him by two and a half centuries in its various fragments. The world he destroyed never came back. The Abbasid caliphate was gone. The Kievan Rus principalities were replaced by a Muscovite tsardom that grew directly from the Mongol administrative model. The Islamic heartland shifted west toward Egypt and eventually toward the Ottoman Empire, which rose partly to fill the vacuum the Mongols had created. When Christopher Columbus sailed in 1492, his stated mission was to reach Cathay, the land of the great khan in China. The Mongol Empire had been gone from China for over a century, but it had so thoroughly defined the European idea of the East that its shadow still pointed the way.