The Battle of Parwan was fought between Sultan Jalal ad-Din Mingburnu of the Khwarezmid Empire and the Mongols ruled by Genghis Khan in September 1221 CE at Parwan, north of Kabul, in present-day Afghanistan. Jalal ad-Din had previously attacked a detachment of Mongols near Wilan, which provoked Genghis Khan into sending an army of 30,000 troops under Shigi Qutuqu. As a result of the tactics adopted by Jalal ad-Din, the Mongol army was destroyed in a two-day battle. As news of the Mongol defeat spread, several cities, including Merv and Herat, which had previously surrendered and accepted Mongol rule, rebelled. In response, Genghis Khan moved to battle Jalal ad-Din, who had lost half of his troops to desertion due to a quarrel over the division of spoils after the battle, and was forced to move to Ghazni to prepare to retreat to India. Genghis Khan intercepted Jalal ad-Din's army as he was preparing to cross the Indus River, and in the ensuing battle he lost his army, treasury and family, but survived to eventually establish a power base in Punjab and Sindh.
Background
Genghis Khan invaded the Khrarezmian 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, Benekat 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. Several Mongol armies invaded Tocharistan, Guzgan and Gharchistan during the last eight months of 1220, collecting a rich plunder in cattle and slaves. 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.