The Khwarazmian dynasty, established in Iran, lasted only until the Mongol invasion under Genghis Khan. By 1218, the rapidly expanding Mongol Empire bordered the Khwarazmian territory. Ala ad-Din Muhammad, the Khwarazmian ruler, had expanded his realm across most of Iran and declared himself shah, seeking recognition from the Abbasid caliph Al-Nasir, which was denied.

The Mongol invasion of Iran began in 1219 after his diplomatic missions to Khwarezm were massacred. The invasion was brutal and comprehensive; major cities like Bukhara, Samarkand, Herat, Tus, and Nishapur were destroyed, and their populations were massacred. Ala ad-Din Muhammad fled and eventually died on an island in the Caspian Sea.

During this invasion, the Mongols employed advanced military techniques, including the use of Chinese catapult units and possibly gunpowder bombs. Chinese soldiers, skilled in gunpowder technology, were part of the Mongol army. The Mongol conquest is believed to have introduced Chinese gunpowder weapons, including the huochong (a mortar), to Central Asia. Subsequent local literature depicted gunpowder weapons similar to those used in China.

The Mongol invasion, culminating in Genghis Khan's death in 1227, was devastating for Iran. It resulted in significant destruction, including the pillaging of cities in western Azerbaijan. The Mongols, despite later converting to Islam and assimilating into Iranian culture, inflicted irreparable damage. They destroyed centuries of Islamic scholarship, culture, and infrastructure, razing cities, burning libraries, and replacing mosques with Buddhist temples in some areas.^[38]

The invasion also had a catastrophic impact on Iranian civilian life and the country's infrastructure. The destruction of qanat irrigation systems, particularly in northeastern Iran, disrupted the pattern of settlements, leading to the abandonment of many once-prosperous agricultural towns.^[39]

Following Genghis Khan's death, Iran was governed by various Mongol commanders. Hulagu Khan, Genghis' grandson, was responsible for further westward expansion of Mongol power. By his time, however, the Mongol Empire had fragmented into different factions. Hulagu established the Ilkhanate in Iran, a breakaway state of the Mongol Empire, which ruled for eighty years and became increasingly Persianized.

In 1258, Hulagu seized Baghdad and executed the last Abbasid caliph. His expansion was halted at the Battle of Ain Jalut in Palestine in 1260 by the Mamelukes. Additionally, Hulagu's campaigns against Muslims caused conflict with Berke, the Muslim khan of the Golden Horde, highlighting the disintegration of Mongol unity.

Under Ghazan (r. 1295–1304), Hulagu's great-grandson, Islam was established as the state religion of the Ilkhanate. Ghazan, along with his Iranian vizier Rashid al-Din, initiated an economic revival in Iran. They reduced taxes for artisans, promoted agriculture, restored irrigation works, and enhanced trade route security, leading to a surge in commerce.

These developments facilitated cultural exchanges across Asia, enriching Iranian culture. A notable outcome was the emergence of a new style of Iranian painting, blending Mesopotamian and Chinese artistic elements. However, after the death of Ghazan's nephew Abu Said in 1335, the Ilkhanate descended into civil war and fragmented into several smaller dynasties, including the Jalayirids, Muzaffarids, Sarbadars, and Kartids.

The 14th century also witnessed the devastating impact of the Black Death, which killed approximately 30% of Iran's population.^[40]