One of the first acts of Ruknuddin's reign was to send an envoy to Yasa'ur Noyan, the Mongol commander camping at Hamdan (the ancient Ecbatana, the Arabs Hamadhan). Yasa'ur replied in this context that Ruknuddin should present himself in person before Halagu, whose arrival was now imminent. This was the first of a long series of messages exchanged in 654/1256 between the Ismailis and the Mongols.
In Zilhaja, 653/January, 1256, Halagu crossed the Oxus and passed the winter in the meadow's of Shafurqan to the west of Balkh. The Russian orientalist Wilhelm Barthold has computed the army of Halagu at about 129,000 men and a thousand Chinese artificers, who were skilled in the construction of military machines and in preparing and using every species of inflamable substances for attacking walled towns and fortified strongholds. Halagu entered Iran through Khorasan in Rabi I, 654/April, 1256 and conquered Tun and proceeded towards Tus. During the Mongol operations, the Ismailis are said to have occupied about 360 mountain castles and strongholds.
In Jamada I, 654/May, 1256, Ruknuddin Khurshah had sent his brother, Shahanshah with a delegation to announce his submission to the Mongols. They met Yasa'ur near Qazwin, and Ruknuddin delegated his own son to accompany the Ismaili mission thence to Halagu. Nine days later, Yasa'ur not only detained Shahanshah, but also invaded the Rudhbar without any reason to demonstrate Mongol's power and attacked the Ismaili forces on a mountain top behind Alamut, but he was forced to withdraw after a short while. He then vacated the whole region upon instructions from Halagu, who had now received Ruknuddin's embassy at Quchan. Halagu professed his satisfaction with Alamut's embassy and his own ambassadors reached Ruknuddin at the end of Jamada II/July and delivered a decree, full of encouragement and benevolence, insisting to demolish his castle and come in person. Ruknuddin did in fact destroy some castles. He also demolished the gates coated with lead and removed the battlements and turrets of Alamut, Lamasar and Maimundiz. The Mongol ambassadors, accompanied by Ruknuddin's envoy Sadruddin returned to report the situation to Halagu. Ruknuddin is said to have asked a year's grace before presenting himself. In the beginning of Shaban/September, the Mongol envoy came with a new proposal that the Ismaili Imam should immediately present before Halagu, and in his absence a Mongol, named Tukel Bahadur would act as a caretaker governor in Rudhbar. Ruknuddin sent his reply through an embassy led by his vizir, Shamsuddin Gilaki and Saifuddin Sultan Malik, who accompanied the Mongol ambassador and reached Halagu on 17th Shaban/September 9, asking for a year's grace and exemption of Alamut and Lamasar from the demolition order, but the Mongol demonstrated their impatience.
Halagu now set out from his encampment near Bastam to launch his assault on the Ismaili strongholds in Rudhbar. The main Mongol force proceeded from different directions. The right wing of Halagu forces led by Buqa Taymur and Koke-Ilgei advanced by way of Mazandaran. The left wing under the Chaghatai prince Teguder and Ket-Buqa proceeded through Simnan and Khuvar. While Halagu himself with the principal army, followed parallel route leading through Firozkuh, Damavand and Ray. He alighted at Damavand for a while and sent yet another message to Ruknuddin. The Imam was asked to come at once to Damavand, and were he to be delayed upto five days by his preparations, he was to send his son in advance. Ruknuddin dispatched his son on 17th Ramdan/October 8. Halagu returned the boy and suggested that if Ruknuddin could not come till later, he should send another brother to relieve Shahanshah. On 5th Shawal/October 26, Ruknuddin sent out his brother Shiranshah with 300 men, who arrived at Halagu's camp two days later.
Meanwhile, the vizir Shamsuddin Gilaki had returned from Girdkuh and brought its governor, the Qadi Tajuddin Mardanshah, before Halagu, while Girdkuh still held out. Shahanshah was relieved and sent back to Rudhbar with the message that if Ruknuddin demolished the castle of Maimundiz and presented himself in a person before Halagu, he would be received with honour and and given immunity. By this time, the Mongol armies entered Rudhbar from all sides. Halagu set out from his base at Piskildara on 10th Shawal/October 31 and advanced towards Rudhbar through Taliqan.