Back in Mongolia Chingis had convened a Khuriltai to plan the invasion of Khwarezm. He was not one to ride off half-cocked. His anger over the murder of his envoy to Otrār had cooled, but his resolution to exact retribution had stiffened. His intelligence networks would have informed him that while the Khwarezmshah was inflicted by infighting among his family and court and by rising discontent among the populace he was still capable of putting half as million or so soldiers into the field. Chingis organized the invasion of Khwarezm in the same step-by-step methodical way he had attacked and finally defeated the Jin in northern China. Leaving the Mongolian Plateau in the spring of 1219, he and his assembled army crossed the passes through the Mongol Altai and dropped down into the upper basin of the Irtysh River, on the northern side of the Zungarian Depression. On the rich grassland straddled the Irtysh he and his men spent the summer fattening their horses. They no doubt also took time to engage in huge hunts for wild game which not only provided food but also served as training exercises for his troops. By the early autumn, when the grass began to yellow, commanders and men were familiarized with each other and their horses were fattened and well-rested. The march west began.
Most accounts imply, even if they do not state outright, that his entire army proceeded en masse to the western end of the Zungarian Basin. (According to one alternative account, Chingis divided his army into two wings, one led by his son Chagatai which would take a northerly route via the Zungarian Basin and another under the command of his son Jochi which would take a southerly route through the Tarim Basin. ) Leaving the bottom of the basin, they rode through the Bor Steppe and by Lake Sayram, areas which as we have seen were in the domains of Ozar Khan and now his son Siqnaq Tegin, and then crossed over the Borohogo Range via the Ak-Tasi Pass.
Then via switch-backed trails they dropped down the great ramparts on the western side of the Borohogo Range into the Ili Valley. (The importance of this route as a gateway from the Zungarian Basin, in current-day Xinjiang Province, China, into the vast Kazakh Steppes behind is underlined by the fact that the China government just recently built a new railway line from Urumqi, the capital of Xinjiang, to the city Korgas, on the Chinese-Kazakhstan border, following this route, with a lengthy tunnel under the Ak-Tasi Pass.)
Just west of Ak-Tasi Pass through the Borohogo Range. Chingis Khan and his army must have come this way.
Ramparts on the western side of the Borohogo Range
Following the Ili River past the last spurs of the mountains they soon emerged out onto the vast steppes of the Seven Rivers Region.
The Ili River
The Ili RiverHere Chingis Khan rendezvoused with his various allies: from Beshbaliq in Uighuristan the The Idikut of the Uighurs, who had sworn allegiance to Chingis back in 1209; from the upper Ili Basin Siqnaq Tegin, the son of the now-deceased Ozar Khan; and already in the Seven Rivers Region the Arslan Khan, (with 6,000 men, according to Juzjani) who had earlier declared his allegiance to Chingis. Juzjani and other Persian historians numbered the assembled multitude at 600,000 men or more, but clearly this was an exaggeration. Barthold, after thoroughly examining all the various and conflicting accounts, concludes that Chingis’s assembled army, including auxiliaries, numbered about 150,000. Juvaini could barely restrain himself in his praises of the Mongol troops:
They were archers who by the shooting of an arrow would bring down a hawk from the hollow of the ether, and on dark nights with a thrust of their spear-heads would cast out a fish from the bottom of the sea; who thought the day of battle the marriage-night and considered the pricks of lances the kisses of fair maidens.
This multitude was now ready to descend on Otrār.