Between 1219 and 1221, the Mongol forces under Genghis Khan invaded the lands of the Khwarazmian Empire in Central Asia. The campaign, which followed the annexation of the Qara Khitai Khanate, saw widespread devastation and atrocities. The invasion marked the completion of the Mongol conquest of Central Asia, and began the Mongol conquest of Persia.
Both belligerents, although large, had been formed recently: the Khwarazmian dynasty had expanded from their homeland to replace the Seljuk Empire in the late 1100s and early 1200s; nearly simultaneously, Genghis Khan had unified the Mongolic peoples and conquered the Western Xia dynasty. Although relations were initially cordial, Genghis was angered by a series of diplomatic provocations. When a senior Mongol diplomat was executed by Khwarazmshah Muhammed II, the Khan mobilized his forces, estimated to be between 90,000 and 200,000 men, and invaded. The Shah's forces were widely dispersed and probably outnumbered—realizing his disadvantage, he decided to garrison his cities individually to bog the Mongols down. However, through excellent organization and planning, the Mongols were able to isolate and conquer the Transoxianan cities of Bukhara, Samarkand, and Gurganj. Genghis and his youngest son Tolui then laid waste to Khorasan, destroying Herat, Nishapur, and Merv, three of the largest cities in the world. Meanwhile, Muhammed II was forced into flight by the forces of Mongol generals Subutai and Jebe; unable to reach any bastions of support, he died destitute on an island in the Caspian Sea. His son and heir Jalal-al Din managed to mobilize substantial forces, defeating a Mongol general at the Battle of Parwan, but these were crushed by Genghis at the Battle of the Indus a few months later.
After clearing up any remaining resistance, Genghis returned to his war against the Jin dynasty in 1223. The war was one of the bloodiest in human history, with total casualties estimated to be between two and fifteen million people. The subjugation of the Khwarazmian lands provided a base for the Mongols' later assaults on Georgia and the rest of Persia; when the empire later divided into separate khanates, the Persian lands formerly ruled by the Khwarazmids would be governed by the Ilkhanate, while the northern cities would be ruled by the Chagatai Khanate. The campaign, which saw the Mongols engage and defeat a non-sinicized state for the first time, was a pivotal moment in the growth of the Mongol Empire.
Background
The dominant power in late twelfth-century Central Asia was the Qara Khitai, which had been founded by Yelü Dashi in the 1130s. Khwarazm and the Qarakhanids were nominally vassals of the Qara-Khitai, but in practice, due to their large population and the extent of their lands, they were allowed to operate almost autonomously.Of these two major vassals, the Qarakhanids were by far the more prestigious; they had ruled in the area for two centuries and controlled many of the richest cities in the region, such as Bukhara, Samarkand, Tashkent and Fergana. By comparison, Khwarazm had only one major city in Urgench, and had only come to prominence after 1150 under Il-Arslan.
However, as the Seljuk Empire slowly fractured after the death of Ahmad Sanjar in 1154, the Khwarazmids were able to take advantage of the chaos due to their geographical proximity; Il-Arslan's son Tekish captured large cities such as Nishapur and Merv in the nearby region of Khorasan, gaining enough power to declare himself a fully-fledged sovereign in 1189. Allying with the Abbasid caliph Al-Nasir, he overthrew the last Seljuk emperor, Toghrul III, in 1194, and usurped the sultanate of Hamadan. Tekish now ruled a great swathe of territory stretching from Hamadan in the west to Nishapur in the east; drawing on his newfound strength, he threatened war with the caliph, who reluctantly accepted him as Sultan of Iran and Khorasan in 1198. The rapid expansion of what was now the Khwarazmian Empire greatly destabilized the Qara-Khitai, which was nominally the overlord. In the early thirteenth century, the khanate would be destabilized further by refugees fleeing the conquests of Genghis Khan, who had begun to establish hegemony over the Mongol tribes.
Muhammad II became Khwarazmshah after his father Tekish died in 1200. Despite a troubled early start to his reign, which saw conflict with the Ghurids of Afghanistan, he followed his predecessor's expansionist policies by subjugating the Qarakhanids and taking their cities, including Bukhara. In 1211, Kuchlug, a prince of the Naimans, managed to usurp the Qara-Khitai Empire from his father-in-law Yelü Zhilugu with Muhammad's help, but alienated both his subjects and the Khwarazmshah with anti-Muslim measures. As a Mongol detachment led by Jebe hunted him down, Kuchlug fled; meanwhile, Muhammad was able to vassalize the territories of Balochistan and Makran, and to gain the allegiance of the Eldiguzids.
Following the defeat of Kuchlug, their shared enemy, relations between the Mongols and the Khwarazmids were initially strong; however, the Shah soon grew apprehensive regarding his new eastern neighbor. The chronicler Al-Nasawi attributes this change in attitude to the memory of an unintended earlier encounter with Mongol troops, whose speed and mobility frightened the Shah. It is also likely that the Shah had grown in pride — like his father, he was now embroiled in a dispute with the Abbasid caliph Al-Nasir, and even went so far as to march on Baghdad with an army, but was repulsed by a blizzard in the Zagros Mountains. Some historians have speculated that the caliph tried to ally with Genghis Khan, especially after Mongol-Khwarazmid relations deteriorated. Mongol historians are adamant that Genghis at that time had no intention of invading the Khwarazmian Empire, and was only interested in trade and even a potential alliance. They cite the fact he was already bogged down in his war against the Jin in China, and that he had to deal with the Hoi-yin Irgen (Tumed) rebellion in Siberia in 1216.
In 1218, the Khan sent a large caravan of Mongol merchants to Khwarazmia; it seems probable that a large proportion of the Mongol elite had invested in the expedition, and thus had a personal interest in its success. However, Inalchuq, the governor of the Khwarazmian city of Otrar, seized the caravan's goods and executed its members on charges of espionage. The validity of the accusations has been debated, as has the Shah's involvement; it is certain, though, that he rejected the Khan's subsequent demands that Inalchuq be punished, going so far as to kill one Mongol envoy and humiliate the other two. This was seen as a grave affront to the Khan, who considered ambassadors "as sacred and inviolable" as the Great Khan himself. He abandoned his war against the Jin, leaving only a small army to pursue it, and gathered as many men as possible to invade Khwarazmia.
Opposing forces
The precise sizes of each force have been heavily disputed; the one certainty is that the Mongol army numbered more than the Shah's. The medieval chronicler Rashid al-Din Hamadani attested that the Mongol army numbered over 600,000 strong, and that they were opposed by 400,000 total Khwarazmians; his contemporary Juzjani gives an even greater estimate of 800,000 for the Khan. These numbers are regarded by modern historians as greatly inflated; the only contemporary source regarded as near-reliable is The Secret History of the Mongols, which gives totals of between 100,000 and 135,000 for the Mongol army, although these totals may have been deflated by a pro-Mongol chronicler.
While Stubbs and Rossabi indicate that the total Mongol invasion force cannot have been more than 200,000, Sverdrup, who hypothesizes that a tumen had often been overestimated in size, gives a minimum figure of 75,000. Most historians have given figures between these two extremes: McLynn estimates the Mongol force at around 120,000; while Smith follows the Secret History with a figure of 130,000. The uncertainty is made worse by the high flexibility and efficiency of the Mongol force's operational structure, allowing it to separate and coalesce at will. As for the Khwarazmians, there is no similarly reliable contemporary source; Sverdrup, taking the proportional exaggeration of the Muslim forces as equal to that of the Mongols, has estimated a total of around 40,000 soldiers, excluding certain town militias. Mclynn however provides a much greater figure of 200,000.
Dispositions
The Khwarazmshah faced many problems. His empire was vast and newly formed, with a still-developing administration. It is known that in 1218 he had overhauled the Seljuk-era administration, replacing it with a streamlined, loyal bureaucracy; the ongoing change may have contributed to disorder during the Mongol invasion. In addition, his mother Terken Khatun still wielded substantial power in the realm; one historian termed the relationship between the Shah and his mother as 'an uneasy diarchy', which often acted to Muhammad's disadvantage.
Additionally, many of the areas that Muhammad charged his troops to defend had been devastated recently by Khwarazmian forces; when later passing through Nishapur, he urged the citizens to repair the fortifications his father had broken down, while Bukhara had been sacked by Muhammad only eight years earlier, in 1212. The Shah also distrusted most of his commanders, with the only exception being his eldest son and heir Jalal al-Din, whose military acumen had been critical on the Irghiz River the previous year. If he had sought open battle, as many of his commanders wished, he would certainly have been greatly outmatched in quantity of troops, let alone quality. The Shah thus made the decision to distribute his forces as garrison troops inside his most important towns, such as Samarkand, Merv and Nishapur.
Genghis' army was commanded by his most able generals, with the exception of Muqali, who was left behind to continue the war against the Jin. Genghis also brought a large body of Chinese siege and construction experts, including several Chinese who were familiar with gunpowder. Historians have suggested that the Mongol invasion had brought Chinese gunpowder weapons, such as the huochong, to Central Asia.
Campaign
Early movements
The Khwarazmshah and his advisers assumed that the Mongols would invade through the Dzungarian Gate, the natural mountain pass in between their (now conquered) Qara-Khitai and Khwarazmian empires. One option for the Khwarazmian defence was to advance beyond the towns of the Syr Darya and block the Dzungarian Gate with an army, since it would take Genghis Khan many months to gather his army in Mongolia and advance through the pass after winter had passed. A Mongol force under Chagatai and Ögedei soon descended onto Otrar from either the Altai Mountains to the north or the Dzungarian Gate and immediately started laying siege to it. Rashid Al-Din stated that Otrar had a garrison of 20,000 while Juvayni claimed 60,000 (horsemen and militia), though like the army figures given in most medieval chronicles, these numbers should be treated with caution and are probably exaggerated by an order of magnitude considering the size of the city.
Unlike most of the other cities, Otrar did not surrender after little fighting, nor did its governor march its army out into the field to be destroyed by the numerically superior Mongols. Instead the garrison remained on the walls and resisted stubbornly, holding out against many attacks. The siege proceeded for five months without results, until a traitor within the walls (Qaracha) who felt no loyalty to the Shah or Inalchuq opened the gates to the Mongols; the princes' forces managed to storm the now unsecured gate and slaughter the majority of the garrison. The citadel, holding the remaining one-tenth of the garrison, held out for another month and was only taken after heavy Mongol casualties. Inalchuq held out until the end, even climbing to the top of the citadel in the last moments of the siege to throw down tiles at the oncoming Mongols and slay many of them in close quarters combat. Genghis killed many of the inhabitants, enslaved the rest, and executed Inalchuq.
Bukhara
At this point, the Mongol army was divided into five widely separated groups on opposite ends of the enemy empire. After the Shah did not mount an active defence of the cities on the Syr Darya, Genghis and Tolui, at the head of an army of roughly 50,000 men, skirted the natural defence barrier of the Syr Darya and its fortified cities, and went westwards to lay siege to the city of Bukhara first. To do this, they traversed 300 miles of the seemingly impassable Kyzyl Kum desert by hopping through the various oases, guided most of the way by captured nomads. The Mongols arrived at the gates of Bukhara virtually unnoticed. Many military tacticians regard this surprise entrance to Bukhara as one of the most successful manoeuvres in warfare.
Bukhara was not heavily fortified, with a moat and a single wall, and the citadel typical of Khwarazmian cities. The Bukharan garrison was made up of Turkic soldiers and led by Turkic generals, who attempted to break out on the third day of the siege. Rashid al-Din and Ibn al-Athir state that the city had 20,000 defenders, though Carl Sverdrup contends that it only had a tenth of this number. A break-out force was annihilated in open battle. The city's leaders opened the gates to the Mongols, though a unit of Turkic defenders held the city's citadel for another twelve days. The Mongols valued artisans' skills highly and artisans were exempted from massacre during the conquests and instead entered into lifelong service as slaves. Thus, when the citadel was taken survivors were executed with the exception of artisans and craftsmen, who were sent back to Mongolia. Young men who had not fought were drafted into the Mongolian army and the rest of the population was sent into slavery in the Mongol Empire. As the Mongol soldiers looted the city, a fire broke out, razing most of the city to the ground.