In 1206 the Mongols numbered perhaps a million people, with an army of around 100,000 riders. To the south lay China, home to perhaps 100 million people, divided among the Jin in the north, the Song in the south and the Tangut kingdom of Xi Xia in the northwest. Within twenty years Mongol armies had ridden from the Yellow River to the Caspian Sea. How?

First targets

Chinggis struck first at Xi Xia, the kingdom of the Tanguts, which controlled trade routes to the west. After a campaign that began in 1209, the Tangut ruler submitted as a vassal, sending tribute and a princess. Then came the Jin, the dynasty that had long played steppe tribes against one another. From 1211 Mongol armies broke through the frontier and ravaged northern China. At first they were poor at sieges; walled cities held out while the countryside burned. They learned quickly, recruiting Chinese and Khitan engineers who built catapults and siege towers. In 1215 the Mongols took Zhongdu, the Jin capital, on the site of modern Beijing, and much of it burned. The Jin court had already fled south. The war in China would go on for two more decades.

The Khwarazmian catastrophe

To the west lay the Khwarazmian Empire of Sultan Muhammad II, a large and recently enlarged Muslim state covering much of Central Asia and Iran. Chinggis seems at first to have wanted trade, not war. In 1218 a caravan of about 450 merchants, sent under Mongol protection, reached the border town of Otrar. Its governor, Inalchuq, had its merchants killed as spies, apparently with the sultan's approval. Chinggis sent envoys to demand justice. The sultan executed one of them and humiliated the others.

The Mongol response, from 1219 to 1221, was among the most destructive campaigns in history. Armies converged from several directions. Bukhara and Samarkand fell in 1220. Juvaini reports that Chinggis rode into Bukhara's great mosque and told its leading men that he was a punishment sent by God for their sins, a scene historians repeat but cannot confirm. Cities that surrendered quickly were usually spared after paying tribute and handing over craftsmen. Cities that resisted, or rebelled after surrendering, faced massacre. Merv, Nishapur and Herat suffered terribly, and Urgench was flooded when the Mongols diverted the Amu Darya river. The sultan fled and died on an island in the Caspian Sea.

How the Mongols fought

Mongol success rested on several advantages. Each warrior brought several spare horses, so he could switch mounts and cover long distances fast. The composite bow, made of wood, horn and sinew, outranged most rivals. Armies moved in separate columns and coordinated across hundreds of kilometers using messengers. A favorite tactic was the feigned retreat, a fake flight to lure pursuers into a trap. Intelligence was thorough: merchants and spies scouted ahead. And the Mongols used terror deliberately. News of massacres spread, persuading other cities to open their gates. Mongol armies also absorbed defeated soldiers: Turkic, Khitan and Chinese troops fought in Mongol campaigns, and in many battles most of the men under Mongol command were not Mongols at all. Several sources report that captured civilians were driven ahead of the army during sieges as human shields. That was the point: mass killing was a policy meant to make other cities surrender, not random rage.

The great raid and the end

While Chinggis returned east, his generals Jebe and Subutai led a reconnaissance in force around the Caspian Sea, through Georgia and across the Caucasus. In 1223, on the Kalka River near the Sea of Azov, they defeated an army of Rus princes and Cuman nomads. A Rus chronicler wrote that no one knew who these strangers were, where they came from or where they went.

Chinggis then turned on Xi Xia, whose ruler had refused to send troops for the western war. During this campaign, in 1227, Chinggis Khan died, probably from illness or from injuries after a fall from his horse; accounts differ. The Tangut capital fell shortly afterward and was savagely punished. Chinggis was buried in secret somewhere in the Mongol homeland, probably near the sacred mountain Burkhan Khaldun. His tomb has never been found, despite many modern searches.

Worked example: the choice before a city

Imagine a city council in eastern Iran in 1220. Mongol envoys arrive with a demand: surrender, tear down your walls, pay tribute, and you will live. The council knows that Bukhara opened its gates and its people survived, though the garrison in its citadel was killed and much of the city burned. It also knows that a neighboring city resisted and was destroyed. Some councilors trust the walls and hope the sultan will return. Others say no help is coming. This is exactly the dilemma the Mongols designed. Their terror worked because their promises were usually, though not always, kept. A modern reader can see a grim logic in it while recognizing what it cost: huge numbers of noncombatants killed to make the next city more pliant. The strategy treated whole populations purely as means to an end, which is what makes it a crime by any humane standard.