Egypt was not the only territory where fighting continued on in spite of the effects of drought, famine, plague and Umar’s policy of consolidation. In Mesopotamia, throughout 638 and 639, there were still pockets of resistance to the Muslim conquests. Yet the most intense fighting came in a hitherto less well-known theatre – the river delta of the Tigris and Euphrates and Khuzestan. After Khalid moved north from Ubulla and won his victory at the Battle of the River in April 633, the fighting in southern Mesopotamia devolved back into the raiding that had preceded his arrival. However, upon his accession in August 634 Umar sent reinforcements to the regional Muslim commander, Suwayd b. Qutba, perhaps with the strategic aim of opening up a second front to divert Persian attention and resources from the middle Euphrates and the movements of al-Muthanna, Jarir and Sa’d.
Surprisingly, this first force under Shurayh b. Amir was destroyed by a Sassanid garrison force near Ohrmazd-Ardashir, modern Ahwaz in the Khuzestan province of Iran. A second force, mostly made up of Thaqif tribesmen, under Utba b. Ghazwan, a veteran of the battles of Badr, Uhud, the Trench and Yamamah, appears to have been of no great size, with the sources suggesting that it was at most 2,000 strong or as small as a mere 40 men. Nevertheless, Utba moved against Ubulla and its garrison of perhaps 500 cavalry in the summer of 635. Camping nearby, he drew the Persians into a fight, defeating them to such an extent that the city fell. Throughout the rest of 635 and into 636, using Ubulla as a base of operations, several Muslim contingents took advantage of what was a complete collapse of Persian authority in the region, with entire districts falling without any real resistance. It was on this captured territory that in 636 Utba was to establish a further forward base along the confluence of the Tigris and Euphrates, called the Shatt al-Arab or `Stream of the Arabs’, that would later become Basra.
The exact timeline of the events after the establishing of Basra is difficult to ascertain but there seems to have been a quick turnover in governors of the new military camp. After Utba died falling from a camel on his way back to Basra after performing the Hajj, the man he had sent to aid Sa’d at Qadisiyyah, al-Mughira b. Shuba, succeeded him as governor. This could have happened as early as late-636/early-637 or as late as 639. Whenever it was, al-Mughira did not last long in his new position for he was removed in order to defend himself from accusations of fornication. Despite being acquitted, he was not restored to Basra, although he did later become the governor of Kufa. al-Mughira’s replacement at Basra was Abu Musa, who oversaw much of the Muslim campaigning into Khuzestan. With the capture of Ubulla and once established at Basra, the next target for the Muslim forces in southern Iraq was the garrison city of Ohrmazd-Ardashir on the east bank of the Karun River, north-east of Basra. By this time it was being defended by a force commanded by Hormuzan, one of the Persian corps commanders at the Battle of Qadisiyyah, who had returned east to defend his own estates in Khuzestan. With an experienced commander now in the field, Persian resistance seems to have stiffened somewhat and local counter-attacks may have briefly driven the Muslims back from Ohrmazd-Ardashir and maybe even back to Basra. However, when the Muslim commander received reinforcements from Sa’d at Ctesiphon and was able to raise more men from the local Arab population, Hormuzan found himself outmatched. A two-pronged Muslim strike towards Ohrmazd-Ardashir forced the Persian commander to retreat across the Karun and, when the Muslim forces then confronted Hormuzan, he agreed to pay the jizya tax in return for a ceasefire that recognised Muslim control of the lands of the Shatt al-Arab and the Karun, essentially cutting the Persians off from the northern reaches of the Persian Gulf.
However, this ceasefire was not meant to last, at least not on Hormuzan’s part. Almost immediately after extricating his army from potential disaster along the banks of the Karun, he was recruiting from amongst the Persian and Kurdish populations to challenge the Muslim hold on Shatt al-Arab. These preparations did not go unnoticed and, after consulting with Umar, Abu Musa led his army back to Ohrmazd-Ardashir. Despite the presence of Hormuzan’s new forces, the Muslims forced a crossing of the Karun and, after a fiercely contested battle, drove the Persians back to Ramhormuz, leaving the Muslims to take Ohrmazd-Ardashir unopposed. A well-pressed pursuit by Abu Musa’s cavalry forced the Sassanids to retire even further east and prompted Hormuzan to again offer a ceasefire, this time in return for recognising the Muslim conquest of Ohrmazd-Ardashir and again Abu Musa accepted.
Once again Hormuzan used this cessation of fighting to build up his forces for another counter-attack. This time he appears to have been directly assured and reinforced by Yazdgerd. However, again Muslim spies brought news of Hormuzan’s imminent treaty-breaking to Abu Musa. A Muslim column then set out from Ohrmazd-Ardashir for Ramhormuz and, when it delivered a sharp rebuke to a Persian force sent to dispute its crossing of the Arbuk River, Hormuzan was obliged to retreat once more and regrouped at the fortress of Tustar to the north. This allowed Abu Musa to occupy not just Ramhormuz but also Izeh, one of the easternmost settlements of Khuzestan.
Yet the strength of the fortifications at Tustar worried Abu Musa. He recommended to the caliph that further reinforcements would be needed if he was to take the fortress. The call was answered not just by the governor of Kufa, Ammar b. Yasir, who at first dispatched 1,000 men under Jarir, only to then lead up to half of his army to Ohrmazd-Ardashir, but from a more unexpected source. Yazdgerd had followed through on his promise to send military aid to Hormuzan; however, he could not have foreseen that a sizeable contingent of those reinforcements would defect to the Muslim camp. Suitably reinforced, Abu Musa and Ammar now marched on Tustar from Ohrmazd-Ardashir, collecting further manpower from the garrisons already established at Ramhormuz and Izeh.
Despite the strength of his fortifications, Hormuzan was confident enough that his forces gathered together at Tustar were capable of defeating Abu Musa’s army. Therefore, as soon as the Muslims arrived outside his walls, he wasted little time in challenging them to open battle, only to suffer another defeat and be forced back into the city. Abu Musa then settled into a blockade, sealing off all routes in and out of Tustar. This siege reportedly dragged on for months, although there is no way to be sure. Finally, running low on supplies, Hormuzan led a desperate sally in an attempt to break out but, in the process of being beaten back, the Persians lost the outer defences of the city. This further demoralisation encouraged an unknown traitor to lead a small band of Muslims through the sewer to open the main gate. With the Muslims flooding into the city, the Sassanids fought valiantly and were able to maintain control of the citadel. However, Hormuzan knew that the situation was hopeless and, the following day, he surrendered himself and the city.
From Tustar, the Muslims advanced to the ancient city of Susa, which was quickly invested. While there were several sallies and assaults, it was to be a gambit using another traitor and a religious prophecy that won Susa for the Muslims. A Persian priest within the city exclaimed from the walls to both defenders and attackers that only a dajjal was fated to capture Susa, a term used by Islamic eschatology in relation to the Day of Judgement with the al-Masih ad-Dajjal – the false Messiah – being similar to the Antichrist of Christianity. However, in a more general sense, the term dajjal means `deceiver’ or `impostor’ and the Persian general, Siyah, who had defected to the Muslim camp in the run up to the siege of Tustar and was present at Susa, claimed that his turning his back on Zoroastrianism in favour of Islam made him a dajjal.
Abu Musa agreed to allow Siyah to try out his proposed ruse. One morning soon after, the Sassanid sentries of Susa noticed a bloodied individual in a Persian officer’s uniform prostrate before the main gate. As there had been a skirmish the previous day, they believed that this man had been left outside the walls overnight and rushed to help him. However, as the gate opened and the sentries approached the fallen officer, he jumped to his feet, revealed himself to be Siyah and slew his intended saviours. Proclaiming that the dajjal had come to conquer the city, the Persian turncoat, along with a group of hidden Muslim soldiers, then charged through the open gate. The Persians attempted to oust them from within the city walls but the attackers were quickly reinforced by regiments of the main Muslim army and the ancient city was soon captured without much resistance.
The capture of Susa left Junde Sabur as the only military position of any importance in Khuzestan unconquered, and while the Muslim attack on it by Aswad b. Rabeea followed the same pattern as many others – the routes in and out of the city were blocked and neither the defenders nor the attackers could force a decisive conclusion before an almost amicable surrender – there was some peculiarity about the ending of the siege. One day, with the blockade still being enforced, the gates of the city opened but, instead of a sally, the inhabitants of the city emerged to go about their daily business. Confused by this, the Muslim forces asked why they thought that hostilities had ceased. The Persian citizens replied that they had accepted their offer of peaceful surrender in return for the payment of the jizya . Aswad contacted the Persian commander to inform him that no such terms had been offered. However, the Persians then produced an arrow complete with a note offering peaceful surrender and a quick inquiry found that a slave from the Muslim army had been responsible.
This left both sides in a quandary as the Persians had surrendered to terms that carried no authority. An uneasy truce prevailed while Aswad sought confirmation on what he should do. Needless to say, Umar was more than happy to reward peace to those who so eagerly sought it as to agree to a slave’s offer. The successful captures of Tustar, Susa and Junde Sabur confirmed Muslim control of not just the entire region from the Persian Gulf to central Mesopotamia but also of the Iranian province of Khuzestan.
Around this time, there is some suggestion that an abortive attempt was made by the Muslim governor of Bahrain, Ula b. al-Hadrami, to conquer a large part of Fars, despite Umar’s order not to invade Iranian territory. Perhaps overconfident and desperate for some personal renown, despite some early success Ula quickly found that his planned attack on Persepolis was beyond his men without reinforcement. Attempting to retreat back to Bahrain, Ula’s army found itself stranded in Fars as the Persians managed to overtake them and burn the boats with which they had crossed the Persian Gulf. Surrounded by superior forces, the Bahraini army was only saved by the timely arrival of a relief force sent from Basra, which then led a prompt withdrawal from Fars.
The exact chronology of these campaigns in Khuzestan and the extent to which this theatre was disrupted by the `Year of Ashes’ and the Plague of Amwas are not altogether clear. However, Tustar, Ramhormuz and Susa would appear to have fallen after the battles of Qadisiyyah and Jalula but certainly before the climactic showdown that was to come at Nahavand in 642. Any attempts to paint a more comprehensive chronological picture than this would `demand more of the sources than they can reasonably be expected to provide’. However, despite the misadventure of Ula, it is fair to say that by the end of 641 the Muslims not only had a firm grip on Syria and Mesopotamia and a strong foothold in Egypt, they also had a launching pad for an attack on the Iranian plateau should they be prompted to use it.