to Islam and remained a useful adviser to Umar throughout the remainder of the Persian campaign. He is also believed to be the mastermind behind Umar's assassination in 644. After the victory at Tustar, Abu Musa marched against the strategically important Susa in January 641, capturing it after a siege of a couple of months. Next, Abu Musa marched against Junde Sabur, the only place left of military importance in the Persian province of Khuzistan, which surrendered to the Muslims after a siege of a few weeks. Battle of Nahavand (642) After the conquest of Khuzistan, Umar wanted peace. Though considerably weakened, the image of the Persian Empire as a fearsome superpower still resonated in the minds of the newly-ascendant Arabs, and Umar was wary of unnecessary military engagement with it, preferring to leave the rump of the Persian Empire alone, commenting, "I wish there was a mountain of fire between us and the Persians, so that neither they could get to us, nor we to them." Persian pride was hurt by the Arab conquest, making the status quo intolerable. After the defeat of the Persian forces at the Battle of Jalula in 637, Yazdgerd III went to Rey and from there moved to Merv, where he set up his capital and directed his chiefs to conduct continuous raids in Mesopotamia. Within four years, Yazdgerd III felt powerful enough to challenge the Muslims again for control of Mesopotamia. Accordingly, he recruited 100,000 hardened veterans and young volunteers from all parts of Persia, under the command of Mardan Shah, which marched to Nahavand for the last titanic struggle with the Caliphate. The Governor of Kufa, Ammar ibn Yasir, received intelligence of the Persian movements and concentration at Nahavand and reported them to Umar. Although Umar had expressed a desire for Mesopotamia to be his easternmost frontier, the concentration of the Persian army at Nahavand forced him to act. He now believed that as long as Persia proper remained under Sasanian rule, the raids into Mesopotamia would continue. Hudheifa ibn Al Yaman was appointed commander of the forces of Kufa, and was ordered to march to Nahavand. Abu Musa was to march to Nahavand from Busra, while Nouman ibn Muqarrin marched from Ctesiphon. Umar decided to personally take the army concentrated at Medina to Nahavand to assume overall command. The members of the Majlis al Shura suggested that Umar should command the