The Mongol campaign against the Nizaris of the Alamut period (the Assassins) began in 1253 after the Mongol conquest of the Khwarazmian Empire of Iran by the Mongol Empire and a series of Nizari–Mongol conflicts. The campaign was ordered by the Great Khan Möngke and was led by his brother, Hülegü. The campaign against the Nizaris and later the Abbasid Caliphate was intended to establish a new khanate in the region—the Ilkhanate.

Hülegü's campaign began with attacks on strongholds in Quhistan and Qumis amidst intensified internal dissensions among Nizari leaders under Imam Ala al-Din Muhammad whose policy was fighting against the Mongols.

In 1256, the Imam capitulated while besieged in Maymun-Diz and ordered his followers to do likewise according to his agreement with Hülegü. Despite being difficult to capture, Alamut ceased hostilities too and was dismantled. The Nizari state was thus disestablished, although several individual forts, notably Lambsar, Gerdkuh, and those in Syria continued to resist. Möngke Khan later ordered a general massacre of all Nizaris, including Khurshah and his family. Many of the surviving Nizaris scattered throughout Western, Central, and South Asia.