The Mongol war machine of horses, tens and borrowed engineers
TL;DR — The Mongols conquered with horse archers on three or four mounts each, a decimal army older than Genghis Khan, relay stations a day apart and siege technicians recruited in China.
The Mongols conquered northern China and Central Asia with an army that the historian David O. Morgan, in the Encyclopaedia Iranica, calls Genghis Khan’s “most impressive creation: organized in the usual steppe fashion on a decimal system, it was a virtually invincible force of cavalry archers (later supplemented by a siege train when cities in China and Persia had to be dealt with).” The decimal system itself was not new; what Genghis Khan changed, the historian Peter Jackson argues, was who commanded its units, and the conquests went on for decades after he died in 1227.
Quick facts
- When
- Genghis Khan proclaimed in about 1206; campaigns against the Tangut Xia (1209), the Jin (1211–1234) and Khwarazm (from 1219); Genghis Khan died in 1227
- Where
- From Mongolia into northern China, Central Asia and Iran
- Who
- Genghis Khan (Chinggis) and his son and successor Ögedei
- Outcome
- Mounted archers in decimal units, a relay network and engineers drawn from conquered peoples carried the conquests on for half a century after his death
Three or four horses per rider
The army’s speed began with its horses. “A cavalryman normally had three or four,” the historian Morris Rossabi wrote, “so that each was, at one time or another, given a respite from bearing the weight of the rider during a lengthy journey.”
The rider’s weapon was a composite bow “made out of sinew and horn.” Rossabi gives its range as “more than 350 yards,” against 250 yards for the English longbow of the same era, and credits “a sturdy stirrup” with steadying the archer’s aim. He quotes a Chinese chronicler on the point: “By nature they [the Mongols] are good at riding and shooting. Therefore they took possession of the world through this advantage of bow and horse.”
Tens and thousands, an old steppe system
Genghis Khan “organized his people into units of ten, a hundred, a thousand, and ten thousand,” in the words of Columbia University’s Asia for Educators. The system was older than he was. Jackson writes in the Encyclopaedia Iranica that “the much-praised decimal system, on which military organization was based, was not peculiar to the Mongols, having been a feature of the Khitan and Jurchen armies.”
The change was in loyalty. Genghis Khan, Jackson explains, removed the ruling elite of tribes that had resisted him and dispersed their people “among new military units,” and “even compliant tribal groups were placed under new commanders whose loyalty was to him alone.”
The army was smaller than its reputation. “Mongol numbers are undoubtedly exaggerated in the majority of sources,” Jackson writes; the figure of 129,000 given by the Persian historian Rashid al-Din “for the entire Mongol army at Čengiz Khan’s death carries greater weight.” One tactic that Asia for Educators calls “particularly effective” was the feigned withdrawal, a pretended retreat that drew pursuers into a trap.
A relay station every day's ride
Orders crossed the empire by relay. “Primarily for military communication,” the OpenStax world history textbook explains, “Chinggis Khan had established a system of horse relay stations called yam on the long-distance roads throughout his realm.” The stations “were located at one-day intervals from one another and included rest areas and supply depots.” His son and successor Ögedei opened the system “to merchants and diplomats.”
Engineers and scribes from the conquered
The Mongols borrowed what they lacked. In China, Jackson writes, “the deployment of large numbers of Chinese infantry and of Chinese siege technicians, in particular, was instrumental in the conquest of those parts of the country unsuited to traditional nomadic cavalry warfare,” and units that deserted from the enemy “played a vital role.”
Government came from the same source. Genghis Khan, Morgan writes, “welcomed the assistance of peoples with greater administrative experience than the Mongols had had, notably the Uighurs of the Tarim basin and the Khitans of Qara Khitay and north China.” The Mongols took their writing from them too: “It was indeed from the Uighurs that Čengīz borrowed a script for Mongolian, previously an unwritten language.”
The Tangut Xia, the Jin, and then the west
The first campaigns went into China. In 1209, Asia for Educators records, Genghis Khan attacked the Tanguts of the Xia state in northwest China after a trade dispute and won “a reduction of the tariffs.” The Jin empire of northern China, Morgan writes, “was first invaded in 1211,” and “by 1215,” according to Asia for Educators, the Mongols had taken the area of modern Beijing. The final conquest of the Jin “was not achieved until 1234, seven years after Čengiz Khan’s death.”
The turn west followed the massacre of a Mongol caravan at Otrar in 1218. Genghis Khan died in the late summer of 1227, on a last campaign against the Xia. He had, in Morgan’s words, “laid the military foundation of an empire which was to continue expanding for a further half century.”
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Key facts
- Each Mongol cavalryman normally had three or four horses (Rossabi).
- The composite bow of sinew and horn had a range of more than 350 yards, against 250 for the English longbow (Rossabi).
- The decimal army of tens, hundreds, thousands and ten thousands was also used by the Khitan and Jurchen armies (Jackson).
- Rashid al-Din’s figure of 129,000 for the whole Mongol army at Genghis Khan’s death “carries greater weight” than larger numbers (Jackson).
- Yam relay stations stood a day’s ride apart; Ögedei opened them to merchants and diplomats (OpenStax).
- Chinese infantry and siege technicians were instrumental in the conquest of China (Jackson).
- The Jin were first invaded in 1211 and finally conquered in 1234 (Morgan).
FAQ
How did the Mongols conquer so much territory?
With mounted archers who rode three or four horses each, a decimal army whose commanders owed loyalty to Genghis Khan alone, relay stations a day apart, and infantry and siege technicians taken from China.
Did the Mongols invent the decimal army?
No. Peter Jackson writes that the decimal system "was not peculiar to the Mongols, having been a feature of the Khitan and Jurchen armies." Genghis Khan's change was to break up tribal groups and place them under commanders loyal to him.
How big was the Mongol army?
Most medieval sources exaggerate it. Rashid al-Din gives 129,000 for the whole army at Genghis Khan's death, a figure Jackson says "carries greater weight."
What was the yam?
A network of horse relay stations on the long-distance roads, set a day's ride apart with rest areas and supply depots. It began for military messages, and Ögedei opened it to merchants and diplomats.
How did the Mongols take walled cities?
With help from the people they conquered. Chinese siege technicians and infantry, Jackson writes, were instrumental where nomadic cavalry could not fight.
Sources
- Encyclopaedia Iranica, David O. Morgan, "ČENGĪZ KHAN"
- Encyclopaedia Iranica, Peter Jackson, "MONGOLS"
- Morris Rossabi, "All the Khan's Horses," Natural History (October 1994), reprinted by Asia for Educators, Columbia University
- Asia for Educators, Columbia University, "Chinggis's Mastery of Organization and Military Tactics"
- Asia for Educators, Columbia University, "Three Invasions Led by Chinggis"
- OpenStax, World History Volume 1, 14.2 "Chinggis Khan and the Early Mongol Empire"
Cite this article
- APA
- Song Letter. (2026, October 11). The Mongol war machine of horses, tens and borrowed engineers. Our Earth's History. https://ourearthshistory.com/east-asia/medieval/how-mongols-swallowed-the-world/
- Chicago
- Song Letter. “The Mongol war machine of horses, tens and borrowed engineers.” Our Earth's History, October 11, 2026. https://ourearthshistory.com/east-asia/medieval/how-mongols-swallowed-the-world/.
- MLA
- Song Letter. “The Mongol war machine of horses, tens and borrowed engineers.” Our Earth's History, 11 Oct. 2026, https://ourearthshistory.com/east-asia/medieval/how-mongols-swallowed-the-world/.
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