By the late 11th century, the Shi'a sub-sect of Ismailism (later Nizari Ismailism) had found many adherents in Persia, although the region was occupied by the Sunni Seljuk Empire. The hostile tendencies of the Abbasid–Seljuk order triggered a revolt by Ismailis in Persia under Hassan-i Sabbah.
Due to the increasingly significant socio-economic issues, the decentralization of the Seljuk government leading to inefficient army mobilization, and a unifying factor of religion in the provinces facilitating the swift spread of the revolt, the Seljuks were unable to quickly put down the revolt.
The conflict was characterized by the weaker Nizaris assassinating key opponents and employing impregnable strongholds, and the Seljuks massacring the Ismailis and their sympathizers.
Due to the Seljuks and Nizaris being unable to complete the war quickly, the Nizaris lost their momentum in the war leading to a stalemate on both sides. Combined with the Nizaris confined to heavily defended castles in unfavorable terrain, the Seljuks reluctantly accepted the independence of the Nizari state.
Sources
The bulk of the sources authored by the Nizaris was lost after the Mongol invasion and during the subsequent Ilkhanate period (1256–1335). Much of what is known about the Nizari history in Persia is based on the hostile Ilkhanate-era history works Tarikh-i Jahangushay (written by the scholar Ata-Malik Juwayni, who was present during the Mongol takeover of the Nizari castles), Zubdat at-Tawarikh (by Abd Allah ibn Ali al-Kashani), and Jami' at-Tawarikh (by Rashid al-Din Hamadani).
Background
In the tenth century, the Muslim World was dominated by two powers: the Fatimid Caliphate ruled over North Africa and the Levant while the Seljuk Empire controlled Persia. The Fatimids were adherents of Ismailism, a branch of Shia Islam, and the Seljuks were Sunni Muslims.
By the final decades of the imamate (leadership of the Ismaili Muslim community) of the Fatimid caliph al-Mustansir Billah (), many people in Seljuk-ruled Persia had converted to the Fatimid doctrine of Ismailism, while the Qarmatian doctrine was declining. Apparently, the Ismailis of Persia had already acknowledged the authority of a single Chief Da'i (missionary) based in a secret headquarters in the Seljuk capital Isfahan. The Chief Da'i in the 1070s was Abd al-Malik ibn Attash, a Fatimid scholar who was respected even among Sunni elites. He led a revolt in 1080 provoked by the increasingly severe Seljuk repressions of the Ismailis.
History
Establishment of the Alamut State
The Ismailis in Persia, including Da'i Hassan-i Sabbah, were aware of the declining power of the Fatimids during the final decades of the imamate of al-Mustansir. Hassan was a new Ismaili convert who had been appointed to a post in the da'wah organization by Ibn Attash in May–June 1072. Within nine years of his missionary activity in several Seljuk provinces, Hassan had evaluated the strengths and weaknesses of the Seljuk military and government and took note of the Seljuk administrative and military prowess. After nine years of intelligence operations, Hassan concentrated his missionary efforts in Daylam, a traditional stronghold for the minority Zaydi Shias which had already been penetrated by the Ismaili da'wa.
Capture of Alamut
By 1087, Hassan had chosen the inaccessible castle of Alamut, located in the remote area of Rudbar (nowadays called "Alamut") in northern Persia, as the base of operations. From Damghan and later Shahriyarkuh, he dispatched several da'is to convert the locals of the settlements in the Rudbar valley near the castle. These activities were noticed by the Seljuk vizier Nizam al-Mulk, who ordered Abu Muslim, the governor of Rayy, to arrest the da'i. Hassan managed to remain hidden and secretly arrived at Daylam, temporarily settling in Qazvin. He was later appointed as the Fatimid Da'i of Daylam.