The Yassa and the Decimal Army Reorganize Mongol Society
A law code and a units-of-ten military system replace tribal loyalty with loyalty to the khan
Quick facts
- Law code
- The Yasa (Yassa)
- Smallest unit
- Arban, 10 men
- Largest standard unit
- Tumen, 10,000 men
- Khan's bodyguard
- Keshig, 10,000 men
What happened
After the 1206 kurultai, Genghis Khan drew up the Yasa (also spelled Yassa), a law code laying out punishments for specific crimes and rules meant to bind Mongols of different tribal backgrounds under one legal order. He paired this with a reorganized army built on a strict decimal structure: units of ten men (an arban), grouped into hundreds (a jagun), grouped into thousands (a minghan), and grouped into tens of thousands (a tumen). The World History Encyclopedia's article on Mongol warfare notes that 98 minghan units of 1,000 men each were created, drawing on tribal levies but deliberately mixing men from different clans so loyalty attached to the khan and his commanders rather than to the old tribal chiefs. Genghis Khan also kept a personal bodyguard of 10,000 men, the keshig, which doubled as a training ground for future commanders of the other divisions.
Why it matters
Breaking the old tribal chain of command was what let the Mongols field armies that answered directly to Genghis Khan rather than to dozens of separate chiefs, and the decimal structure gave the army a scalable, standardized organization that outlasted individual campaigns. This is the military and legal machine that made the conquests of Western Xia, the Jin, and Khwarazm possible within a single generation.
How we know
The decimal military structure and the Yasa's existence are described in the Secret History of the Mongols and later Persian sources such as Juvaini and Rashid al-Din; the World History Encyclopedia's Mongol Warfare article synthesizes these accounts.
Sources
- World History Encyclopedia. Genghis Khan · Reputable sourceworldhistory.org · The domain "worldhistory.org" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- World History Encyclopedia. Mongol Warfare · Reputable sourceworldhistory.org · The domain "worldhistory.org" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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