Jalal ad-Din had previously attacked a detachment of Mongols near Wilan (Waliyan), which provoked Genghis Khan into sending an army of 30,000 troops under Shigi Qutuqu.

[6] Genghis Khan commanded a skilled, disciplined, combat-proven army of 150,000 to 200,000 soldiers,[7] mostly Mongols and other allied tribes who were well-drilled in their method of warfare.

[14] The mistrust that the Shah had for his Qanqli Turk troops and commanders[15][8] meant he could only offer battle under favorable conditions with superior numbers.

He adopted a defence in depth strategy based on fortified cities,[16][7][17][18] and stationed garrisons of veteran soldiers at various cities including Otrar, Bukhara, Banakat and Samarkand, trusting to the Mongol inexperience with siegecraft[19] and their unfamiliarity with the terrain to delay their progress and give him the chance to offer battle at his own initiative.

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.

[24][25] The rapid fall of Transoxania further unnerved Shah Muhammad II, who began to retreat west along with Jalal al-Din.

[28] The Mongol army sacked several cities, including Zaveh, Quchan, Tus, Qazvin and Ardabil,[29][22] and then wintered in the Mughan steppes.

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[31] to reach Nishapur.

[34] Jalal al-Din spent the summer of 1221 in Ghazni where thousands of people from all over Afghanistan joined his ranks to defend their homelands after hearing the fate of Bamiyan.

[37] 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.

[37] Shigi Qutuqu was overconfident after the continuous Mongol successes, and he quickly found himself on the back foot against the more numerous Khwarazmian force.

[42] When Genghis Khan heard the news of the defeats, he made forced marches to catch Jalal al-Din before he escaped into India.

[50][38] According to Rashid al-Din Hamadani's account, Genghis Khan personally visited the battlefield and the fallen Mongol soldiers, and addressing Shigi Qutuqu, said:[51]You did not know the place of battle, and you were both at fault.The Khwarezmians started an insurgency after the news of Shigi Qutuqu's defeat at the battle of Parwan spread throughout the empire.

An insurgency leader named Muhammad the Marghani twice attacked Genghis Khan's camp at Baghlan and returned with loot.

Cities that had peacefully surrendered rose up in arms, which forced Genghis and his son Tolui to spend extra months subduing the revolts.