The capture of such an ancient city as Susa, along with a considerable quantity of booty and artefacts such as the coffin of Daniel, considered a prophet by Islam, seems to have been viewed by many Muslims, including Umar, as the culmination of the war with the Persians. However, while Umar displayed some reticience in continuing the war, Yazdgerd did not. As his empire had been greatly humiliated and weakened by the loss of Mesopotamia and Khuzestan, rather than accept the lull offered by Umar and take time to stabilise and consolidate his position in Iran – as Heraclius had done in Anatolia – Yazdgerd could not abide a cessation of the war.
While the most obvious reasons for this continuation were how injurious the loss of Mesopotamia and Khuzestan had been to the Sassanid state both in terms of prestige and material resources, perhaps the most pressing was the weakness of Yazdgerd’s own position. He had come to the Sassanid throne during a period of relentless civil war and, while he had eventually become recognised as the sole ruler of Persia, he did not enjoy the same autocratic control over his empire as many of his predecessors had enjoyed. The power and influence of Persian generals, governors and warlords therefore obliged Yazdgerd to challenge the Muslim conquests as quickly as possible before his own right to rule was challenged from within. In the simplest terms, Yazdgerd had to risk the continuation of war with the Muslims and the potential for destruction it entailed in order to prevent the Persian state from descending back into the anarchy that had plagued it since the death of Khusro II.
Having retreated to Qom, Yazdgerd put out a call to all his remaining forces to congregate at Nahavand, about fifty miles south of Ecbatana, the modern Iranian city of Hamadan. The size of the forces that gathered there is variously recorded as anything from 50,000 to 100,000. While such large numbers are usually cause for concern and the forces available to the Persians will have been drastically reduced by the wars of the seventh century, that Yazdgerd portrayed the upcoming battle as being for the future of the Sassanid state and its Zoroastrian faith means that it is entirely possible that he was able to bring together a vast host from across all of Persian society – soldiers, clergy, farmers, artisans and peasantry. It is therefore possible that the Persian army that was to fight at Nahavand was in the order of several tens of thousands.
However, such a congregation of troops did not go unnoticed by the Muslims scouts deployed in the frontier lands and word soon reached Ammar at Kufa and then Umar himself that the Persians were planning a massive attack. Suspecting that the Persians planned to attack Basra and Kufa, Ammar advised the caliph that they should gather together the bulk of their forces and attack the Persians whilst they were still congregating at Nahavand. In Medina, Umar sought the advice of his council and, while they all agreed that a preemptive strike against Nahavand was the best course, there was some disparity on what forces should be sent. Uthman proposed the most drastic action of gathering the full might of their forces and sending them against Yazdgerd, although Ali suggested that such a move would only provoke extensive counterattacks from the Romans. Therefore, he suggested that the forces of Kufa, Basra and along the frontier, reinforced by a levy of new recruits and veterans, would be enough to defeat Yazdgerd if a surprise attack was launched. Umar agreed with the assessments of both Ammar and Ali, issuing a call to arms from amongst the peoples of Arabia. He was also swayed from leading the attack on Nahavand in person and appointed Nu’man b. Muqrin, a subordinate of Sa’d, to command the combined Muslim forces.
Collecting together the Muslim forces posted on the frontier, Nu’man advanced to Tazar, a few miles west of Kermanshah, to await the arrival of the forces from Basra, Kufa and Medina. This coming together was seemingly completed by the beginning of December 641 and seems to have provided Nu’man with up to 30,000 men. The Muslims then set about scouting the land ahead of them to see if the Persians had noted their mobilisation. When the report came back that there were few or no Persians between the Muslims and Nahavand, Nu’man ordered a rapid advance to Isbeezahan, a small town about ten miles from Nahavand. The land that lay between the Muslim and Persian forces was framed by high ridges to the north-east and south-west with the gap between these two dominated by a stream and a smaller outcrop known as the Brown Ridge. Once they realised that the Muslims were upon them, the Persian commander, Bahman, quickly established this ridge and the stream as the backbone of his dispositions, deploying his forces in an `L’ shape in order to follow the path of the stream and to have the ridge to their rear. The Persian lines were anchored on their right by the town of Darizeed and on their left by the divergence of the stream and the town of Zarrameen. Bahman was also careful to place a large number of caltrops on the shore of the stream to impede the Muslim cavalry.
It is possible that his advantage in numbers and the strength of his position may have made Bahman somewhat overconfident, as he was willing to allow the Muslim forces to deploy in front of his men unhindered. However, it is also possible that he was more than aware of the events of Jalula and how the Persians had suffered defeat by leaving a well-fortified position. Nu’man took up the challenge laid down by Bahman and deployed his men opposite the Persian lines with his brother, Nueim, taking command of the section of the Muslim line between Darizeed and Isbeezahan; Nu’man himself commanded the Muslim centre, the left flank of which began near Isbeezahan, and the Muslim right that reached to a position opposite Zarrameen was commanded by Hudayfah b. al-Yaman.
Relying on the strength of Muslim morale and the momentum of repeated victories, Nu’man launched an all-out attack across the stream along the entire length of his battle lines. However, the combination of the stream, the ridge and the caltrops disrupted the Muslim attack, allowing the Persian lines to stand firm throughout the first day of fighting. The second day played out along similar lines with neither side able to force any kind of breakthrough and the only tangible results being the casualties suffered by both sides. While it is easy to be critical of Nu’man for such unimaginative attacks in the opening forty-eight hours of the fighting, this was a testament to the defensive position that Bahman had been able to take up in spite of the lack of preparation that the Muslim advance on Nahavand had given him.
Recognising the strength of the Persian position, Nu’man now took a more passive stance, daring the Persians to leave the safety of their defences to attack his army. However, Bahman refused to be drawn in to such a rash attack, determining instead to use the increasingly cold weather to further sap the Muslim strength. For the next two days there was little fighting aside from some minor Persian raiding parties. Mindful that time was not on the side of their army, as the Persians were using the proximity of Hamadan to receive reinforcements and supplies, the Muslim leadership decided on the same risky tactic that had worked so well at Jalula – the feigned retreat. However, perhaps wary that Bahman might not take the bait, the Muslims added an extra layer to this ruse. The rumour was put about that Umar had died and was given perhaps a week to seep into the Persian camp. At the end of that week, the forces of Nu’man and Hudayfah withdrew north-west to join Nueim at Isbeezahan.
Buoyed by the rumour of Umar’s passing, the week of Muslim inactivity, and now their retreat, Bahman felt the time was right for a decisive counter-attack. Leaving the security of his defences and gathering his forces opposite Isabeezahan, the Persian commander ordered his men to advance after the retreating Muslim forces. However, as soon as the Persians moved onto the obstacles of the stream and their own caltrops, Nu’man ordered his forces to turn and attack. As with the first days of the battle, it was a brutal and bloody contest, one that claimed the life of Nu’man himself. Hudayfah’s quick assumption of command meant that there was little disruption to the Muslim force; although, while the Muslims do seem to have pushed the Persians back somwhat, there was no decisive breakthrough throughout the vast majority of the daylight hours.
However, before he had been incapacitated, Nu’man had put in place the mechanisms for a Muslim victory. He knew that as the Persians advanced on Isbeezahan they would not only move away from their defences around the Brown Ridge, they would also detach their right flank from the security of Darizeed and its surrounding hills, exposing it to an attack from the north-east. To take advantage of this, Nu’man had gathered together his cavalry and sent it under the command of Qaqa to hide in the foothills north-east of Isbeezahan, a move that went unnoticed by Bahman. Now, as the sun began to set on a hard day’s fighting, Qaqa struck hard at the Persian right whilst Hudayfah and Nueim ordered a renewed assault along the entire Muslim line. Attacked head on, in the flank and disrupted by the stream and caltrops, the Persian line began to pull back. The death of Bahman in the early stages of the retreat proved to be fatal as the large numbers of raw recruits meant that any sort of ordered withdrawal under concerted Muslim pressure was impossible. As they had always done, the Muslims pressed their advantage to the fullest. The next day Hudayfah took most of his force against the now isolated garrisons at Darizeed and Nahavand while Nueim and Qaqa led a contingent after those retreating towards Hamadan, which quickly surrendered on terms.