Genghis Khan invaded the Khwarazmian Empire to avenge the murder of a Mongol trade caravan by the government of
Otrar and the subsequent refusal of Shah
Ala ad-Din Muhammad II to bring the governor of Otrar to task for his crime. Genghis Khan commanded a skilled, disciplined, combat-proven army of 150,000 to 200,000 soldiers, mostly Mongols and other allied tribes who were well-drilled in their method of warfare. The army also included a corps of Chinese
siege engineers. Genghis Khan was a charismatic, inelegant and experienced leader, his sons
Jochi,
Chagatai,
Ogedei and
Tolui were competent generals, and he was also served by brilliant generals like
Jebe and
Subutai, adept in employing flexible and innovative tactics. Shah Muhammad II may have been able to mobilize a mercenary army numbering 200,000 to 400,000 men, but his Turkish soldiers were undisciplined, and unity was lacking between the Turks, Iranians, Arabs and Afghans in the army. The mistrust that the Shah had for his
Qanqli Turk troops and commanders meant he could only offer battle under favorable conditions with superior numbers. He adopted a
defence in depth strategy based on fortified cities, and stationed garrisons of veteran soldiers at various cities including
Otrar,
Bukhara,
Banakat and
Samarkand, trusting to the Mongol inexperience with siegecraft and their unfamiliarity with the terrain to delay their progress and give him the chance to offer battle at his own initiative. He planned to raise a new army beyond the
Amu Darya near Kelif and then strike the Mongols in
Transoxania, or defend the Amu Darya barrier by preventing the Mongols from crossing the river, and if needed retreat to
Ghazni and then to India. Genghis Khan invested Otrar with his entire field army in September 1219. After some time, he divided his army, sending a detachment under his eldest son Jochi down the Syr Darya, and another division to march on
Banakat. Leaving Chagatai and Ogedei to maintain the siege of Otrar, Genghis Khan and Tolui crossed the
Kyzylkum Desert to attack
Bukhara, which fell in February 1220, and Samarkand, which was taken in March 1220. Banakat was also occupied, Otrar fell in April 1220, and the Mongol armies from Banakat and Otrar joined Genghis Khan near
Nasaf, where they spent the summer of 1220 resting the army and the horses. Jochi had taken all the towns along Syr Darya, including
Sighnaq and
Jend, by April 1220, then camped on the Kipchak steppes. Genghis Khan sent a 30,000–40,000 man army led by
Jebe and
Subutai and his own son-in-law Toghachar to hunt down the Shah. The rapid fall of Transoxania further unnerved Shah Muhammad II, who began to retreat west along with Jalal al-Din. He had halted for a while at
Nishapur, but when the Mongol army under
Jebe and
Subutai crossed the Amu Darya, the Shah moved across Persia, then eluded the Mongols by pretending to make for
Baghdad, and eventually found refuge on an island in the Caspian Sea, where he died in December 1220, naming Jalal ad-Din his heir. The Mongol army sacked several cities, including
Zaveh,
Quchan,
Tus,
Qazvin and
Ardabil, and then wintered in the
Mughan steppes. Jalal al-Din needed an army to confront the Mongols. The Sultan went to
Gurganj, a city reportedly housing 90,000 troops, but the city officials preferred his brother Uzlaq Shah as the Sultan. After discovering a plot against his life, the Sultan with 300 cavalry crossed the
Karakum Desert in 16 days and defeated a Mongol detachment near
Nisa to reach Nishapur. Jalal al-Din intended to raise an army at
Nishapur, but abandoned the city when Mongols arrived unexpectedly. The Mongols chased the Sultan across Khuistan, but Jalal al-Din managed to elude his enemies to reach
Bost. Here, an army of 10,000 Turks commanded by his maternal uncle Amin Malik joined him, and the Sultan reached Ghazni after driving off a Mongol army from Qanhahar after a three-day battle.
Prelude '', Rashid al-Din, 1410-1430. Jalal ad-Din regrouped at Parwan. A week later, Genghis Khan sent his chief justice
Shigi Qutuqu to hunt down Jalal al-Din, but only gave the inexperienced general 30,000–50,000 troops. Medieval scholarship differed on the strength of the Mongol army.
Ata Malik Juvayni reported Shigi Qutuqu's strengths as 30,000,
Juzjani reported it as 45,000, and Ibn Abd Allah al-Umari reported it as 70,000. Juzjani, Nasawi, Juvayni,
Ibn al-Athir and Handmir all report the Khwarazmian strength as 60,000. However, modern scholarship differs on the strength of both armies. The lowest estimate for Jalal al-Din's strength is 30,000, while the highest is 120,000. In
The Harper Encyclopedia of Military History, Richard and Trevor Dupuy give Jalal al-Din's force as 120,000. Tucker similarly gives Jalal al-Din's strength as 120,000. Estimates for Shigi Qutuqu's strength range between 30,000 and 70,000. Mclynn Frank estimates the Mongol forces were around 45,000–50,000, whereas he estimates Jalal al-Din had 60,000–70,000; he further adds that while the numbers are exaggerated, the proportion of Jalal al Din's army's numerical superiority is probably accurate. On the other hand, Carl Sverdrup assesses that Jalal al-Din probably had 15,000 men in total whereas Shigi Qutuqu commanded as many as 10,000 men. The larger Khwarazmian army was ill-equipped and consisted mostly of infantry, whereas Shigi Qutuqu's entire army was well-equipped cavalry. == Battle ==