How did one murdered caravan bring the Mongols west?

It began with a caravan. In 1218 Genghis Khan sent a large group of Mongol merchants to Khwarazmia, an empire in Central Asia. Inalchuq, the governor of the city of Otrar, seized their goods and executed them on charges of espionage. Historians debate whether the charge was true and how far the Shah was involved, but the Shah refused to punish the governor, killed one Mongol envoy and humiliated the other two. Historians of the Mongols are adamant that Genghis had no intention of invading and wanted trade, even an alliance. After the envoys' treatment, he came anyway.

The Shah chose to spread his forces as garrisons in his main towns, such as Samarkand, Merv and Nishapur, because he distrusted most of his commanders. That let Genghis defeat them one after another. He crossed 300 miles of the Kyzyl Kum desert between oases, guided by captured nomads, and arrived at the gates of Bukhara virtually unnoticed. At Samarkand he sent deserters with letters implying that the Shah's mother and some generals were on his side, which inflamed divisions, and then drew half the garrison out of the walls with a feigned retreat. The Shah died on a small island in the Caspian Sea in December 1220.

The cost was enormous. Genghis and Tolui laid waste to Khorasan, destroying Herat, Nishapur and Merv, three of the largest cities in the world. Medieval chroniclers reported 700,000 to 1,300,000 deaths at Merv and 1.2 million at Gurganj; historians call the Gurganj figure almost certainly exaggerated, though its sack is considered one of the bloodiest massacres in human history. The campaign was also a milestone: it was the first time the Mongols engaged and defeated a non-sinicized state, and a pivotal moment in the growth of the Mongol Empire.