Mongol campaign against the Nizaris participant Mongol Empire
The Mongol campaign against the Nizaris of the Alamut period (the Nizari Ismaili state) began in 1253 after the Mongol invasion of the Khwarazmian 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 Muhammad III of Alamut whose policy was fighting against the Mongols. His successor, Rukn al-Din Khurshah, began a long series of negotiations in face of the implacable Mongol advance. 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. Little is known about them afterward, but their communities maintain some sort of independence in their heartland of Daylam and their Imamate reappear later in Anjudan.Early Nizari–Mongol relations
In 1221, the Nizari Imam Jalal al-Din Hasan sent emissaries to Genghis Khan in Balkh. The Imam died in the same year and was succeeded by his 9-years-old son, Ala al-Din Muhammad.
After the fall of the Khwarezmian dynasty as a result of the Mongol invasion, direct confrontation began between the Nizaris under Imam Ala al-Din Muhammad and the Mongols under Ögedei Khan. The latter had just begun to conquer the rest of Persia. Soon the Nizaris lost Damghan in Qumis to the Mongols; the Nizaris had recently taken control of the city after the fall of the Khwarezmshahs.The Nizari Imam sought anti-Mongol alliances as far as China, France, and England: in 1238, he and the Abbasid caliph Al-Mustansir sent a joint diplomatic mission to the European kings Louis IX of France and Edward I of England to forge a Muslim–Christian alliance against the Mongols, but this was unsuccessful. The European kings later joined the Mongols against the Muslims.In 1246, the Nizari Imam, together with the new Abbasid caliph Al-Musta'sim and many Muslim rulers, sent a diplomatic mission under Nizari muhtashams (governor) of Quhistan Shihab al-Din and Shams al-Din to Mongolia on the occasion of the enthronement of the new Mongol Great Khan, Güyük Khan; but the latter dismissed it, and soon dispatched reinforcements under Eljigidei to Persia, instructing him to dedicate one-fifth of the forces there to reduce rebellious territories, beginning with the Nizari state. Güyük himself had intended to participate but died shortly afterward. A Mongol noyan (commander), Chagatai the Elder, was reportedly assassinated by the Nizaris around this time.Güyük's successor, Möngke Khan, began to implement the former's schemes. Möngke's decision followed anti-Nizari urges by Sunnis in the Mongol court, new anti-Nizari complaints (such as that of Shams al-Din, qadi of Qazvin), and warnings from local Mongol commanders in Persia. In 1252, Möngke entrusted the mission of conquering the rest of Western Asia to his brother Hülegü, with the highest priority being the conquest of the Nizari state and the Abbasid Caliphate. Elaborate preparations were made, and Hülegü did not set out until 1253, and actually arrived in Persia more than two years later.  In 1253, William of Rubruck, a Flemish priest sent on a mission to Karakorum in Mongolia, was struck by the security precautions there, reportedly in response to the more than forty assassins who had been sent there to assassinate Möngke; it is possible that the assassination attempt was merely rumored.Campaign against Rudbar and Alamut
As soon as he had been in power, Khurshah announced the Nizari leadership's willingness to submit to the Mongol rule to the nearest Mongol commander, noyan Yasur in Qazvin. Yasur replied that the Imam personally should visit Hülegü's camp. Fightings are recorded between Yasur and the Nizaris of Rudbar: on June 12, he was defeated in a battle on Mount Siyalan near Alamut, where the Nizari forces had been mustered, but managed to harass the Nizaris of the region.As Hülegü reached Bistam, his army had enlarged into five tümens, and new commanders were added. Many of them were the relatives of Batu Khan. From the ulus of Jochi representing the Golden Horde came Quli (son of Orda), Balagha, and Tutar. Chagatai Khanate forces were under Tegüder. A contingent of Oirat tribesmen also joined under Buqa Temür. No member of Ögedei's family is mentioned. Hülegü had with him a thousand squads of siege engineers (probably north Chinese, Khitan and Muslim) skilled in the use of mangonels and naphtha.
The Mongols campaigned against the Nizari heartland of Alamut and Rudbar from three directions. The right wing, under Buqa Temür and Köke Ilgei, marched via Tabaristan. The left wing, under Tegüder and Kitbuqa, marched via Khuwar and Semnan. The center was under Hulegu himself. Meanwhile, Hülegü sent another warning to Khurshah. Khurshah was in Maymun-Diz fortress and was apparently playing for time; by resisting longer, the arrival of winter could have stopped the Mongol campaigning. He sent his vizier Kayqubad; they met the Mongols in Firuzkuh and offered the surrender of all strongholds except Alamut and Lambsar, and again asked for a year's delay for Khurshah to visit Hülegü in person. Meanwhile, Khurshah ordered Gerdkuh and fortresses of Quhistan to surrender, which their chiefs did, but the garrison of Gerdkuh continued to resist. The Mongols continued to advance and reached Lar, Damavand, and Shahdiz. Khurshah sent his 7- or 8-years-old son as a show of good faith, but he was sent back due to his young age. Khurshah then sent his second brother Shahanshah (Shahin Shah), who met the Mongols at Rey. But Hülegü demanded the dismantling of the Nizari fortifications to show his goodwill.Numerous negotiations between the Nizari Imam and Hülegü were futile. Apparently, the Nizari Imam sought to at least keep the main Nizari strongholds, while the Mongols were adamant that the Nizaris must fully submit.