Caliph Umar (Omar) dispatched Sa’d ibn Abi Waqqas and a new Arab 30,000-man army against Persia. Sa’d defeated a Sassanid Persian force of 50,000 men under Rustam in the Battle of al-Qadisiyyah during November 16-19, 636. This strategically significant encounter led to the Muslim capture of the Persian capital of Ctesiphon several months later. Sa’d and the Arabs defeated the Persians again in the Battle of Jalula in December 637.
During 637-645 the Arabs completed the conquest of Syria and Palestine. Among prominent places taken were Jerusalem and Antioch (638), Aleppo (639), Caesarea and Gaza (640), and Tripoli (645). Most fell after lengthy sieges. At the same time, during 639-641 the Arab forces conquered all of remaining Byzantine Mesopotamia.
Egypt was the next Muslim target, beginning in 639. In July 640, General Amir ibn al-As was victorious over Byzantine forces in the Battle of Babylon, near Helliopolis. Following long sieges, he captured the fortified cities of Babylon in April 641 and Alexandria in September 642.
During 640-650 the Arab armies conquered what remained of Persian territory. Following decisive Muslim Arab victories in the Battle of Ram Hormuz in 640 and the Battle of Nahavend in 641, organized Persian resistance came to an end. During the next decade the Arabs solidified their control over what had been the Sassanid Persian Empire, with the Oxus River the boundary between Arab and Turkish territory.
During 642-643 the Muslims expanded into North Africa from Egypt. Under Abdullah ibn Zubayr, they captured Cyrene and Tripoli, then raided farther west. In 645 Muslim forces under Amr turned back an ineffectual Byzantine effort to recapture Alexandria. A revolt within the city, however, forced Amr to retake Alexandria by storm.
Battle of Jalula
This lack of an organised defence of their capital Ctesiphon not only demonstrates the poor state to which the Persian military had fallen through its defeats by Romans, Turks, civil war and now Muslim Arabs, but also how unprepared the Persian defences of Ctesiphon were for an attack from the south. Centuries of warfare against the Romans and the nomadic tribes of the Eurasian steppe had concentrated Persian defensive efforts to the north of Ctesiphon. The contrast between the destruction of the bridges over the Nahrawan canal to block Heraclius’ approach in 627 and the ease with which Sa’d approached Veh-Ardashir and then took Ctesiphon in 637 demonstrates the direction in which Persian defences were facing. It could be argued that, by leaving troops in Mesopotamia to slow the advance of the Muslims on Ctesiphon, Yazdgerd assured the capture of his capital by depriving its defence of much needed manpower. However, without garrisons at the likes of Burs, Babylon and Kusa, Zuhra’s advanced guard would have arrived at Ctesiphon before any defensive measures were implemented. Therefore, after the defeat at Qadisiyyah, the Sassanid king and his generals were left with what was a no-win situation with regard to defending Ctesiphon.
However, this Persian evacuation of their capital without a fight meant that there were still sizeable Sassanid armies in the field that needed to be defeated before Muslim control of Mesopotamia could be consolidated. The main Persian force under Mihran and Khurrazad retreated north to Jalula, which, as well as being near the modern site of Baghdad, lay on a strategically important route between the Persian provinces of Mesopotamia, Khurasan and Atropatene. There were also forces congregating to the north at Birtha, usually identified with modern-day Tikrit, as well as the significant garrison of the fortress further up the Tigris recognised as modern Mosul. Its governor, Intaq, appears to have moved south to Birtha with his garrison and along with some survivors from Ctesiphon and new recruits from the local Arab tribes formed a sizeable force.
The relative proximity of Birtha to the main Sassanid force at Jalula meant that Intaq could move to join his forces to those of Mihran and Khurrazad as well as providing a potential route of retreat for the Persian force should it be defeated at Jalula. Therefore, whilst Sa’d sent the majority of his force against Jalula under Hashim in April, he also sent about 5,000 men under Abdullah to preoccupy if not neutralise Intaq. Upon arriving, Abdullah attempted to storm the walls with a lightning attack. However, Intaq’s men held firm and it appears as though Abdullah became concerned about the size of the garrison. To deal with this perceived strength, the Muslim commander attempted to drive a wedge between the elements of Intaq’s force. Muslim spies made contact with the Christian Arab contingent and persuaded them to side with Abdullah rather than Intaq. The Persians seem to have gotten wind of this betrayal or at least suspected it, as they attempted to abandon Birtha along the river. However, they found themselves trapped between the attacking Muslims and their former Arab allies and the Persian garrison was quickly overrun. A few days later, a small Muslim force received the surrender of Mosul without much of a fight.
While Abdullah was cutting off a potential route of retreat and reinforcement for the Sassanids, Hashim had squared up to the Persian forces at Jalula. While the strategic position of Jalula as a crossroads for the Sassanid state meant that it was vital for Mihran and Khurrazad to try to defend it, the position of the town with the Diyala River to the west and foothills of the Zagros Mountains to the east also offered an excellent defensive position. Knowing that the naturally narrow plain in front of Jalula would funnel the Muslim army towards the town and protect their flanks, Mihran prepared diligently for the Muslim attack he knew would come. Jalula itself was turned into a fort, protected by a line of trenches stretching from the broken ground of the Zagros foothills to the Diyala and caltrops to further hinder the Muslim infantry and cavalry. Archers and artillery were also positioned on the fortifications to bleed the Muslims as they approached the walls. Only after inflicting crippling damage on the Muslim ranks would Mihran then leave this defensive position in order to win a decisive victory.
Upon surveying the disposition and defences of the Persian force, Hashim recognised Mihran’s ploy in presenting the Muslims with only one offensive option – a costly frontal assault. This was something that he could ill afford given that the size of the forces arrayed at Jalula were likely very similar, around 12,000 each. Therefore, Hashim decided to draw the Persians away from their defences by employing one of the riskiest manoeuvres in battlefield tactics – the feigned retreat. The danger of this tactic is that a feigned retreat can quickly become an actual one if the morale and discipline of those attempting it is not strong enough and a counter-attack from the opponent is so well pressed and coordinated as to be impossible to resist. Clearly, after the numerous victories they had won up to the battlefield of Jalula, Hashim had every reason to believe in the discipline and prowess of his men to even attempt such a tactic. While there is no evidence to suggest that Mihran’s counter-attack was not well pressed, it could be argued that the presence of their own trenches and caltrops could have prevented the Persians from launching a fully coordinated assault on the `retreating’ Muslims as they had to waste time in placing a bridge over the defences.
The battle therefore began with a Muslim attack on the defences of Jalula, only for them to retreat under the hail of Persian archers and artillery. Mihran took this as a sign that his plan was working and that the Muslim forces were on the verge of breaking and quickly launched his planned counter-attack. Unbeknownst to the Persian commander, his opposite number will have also been pleased that his own plan was going well. His men had fooled the Persians into thinking they were retreating whilst still retaining their own discipline and order. With the Persians now drawn away from their defences, an infantry confrontation took place on the plain before Jalula. Further staged withdrawals by Hashim’s men then opened up a gap between the Persian lines and the bridge route back into the fort and it was then that Hashim launched his counterstroke. Having gathered together a strong cavalry contingent in his rear under Qaqa, Hashim now sent them in an attack around the Persian right flank against the lightly defended bridge. Once word filtered through the battlefront that the Muslims had cut off the only escape route, Hashim ordered his men in a full-scale attack on the Persian lines while Qaqa attacked their rear. Trapped by geography, their own defences and the Muslim forces, the Persian army broke. Despite many men making it back to the fort of Jalula, the defeat of Mihran and the death of Khurrazad had neutralised it as a threat. The exact date for the Battle of Jalula is difficult to pin down from the sources, some of which place the battle at the end of a seven-month siege while others say that the seven-month siege succeeded a battle in April 637.
Whatever the order of events, Jalula had fallen to Hashim by the end of 637. The Muslim general then sent Qaqa after those Persian forces under Mihran who had managed to escape. The cavalry commander caught up to them at the city of Khanaqin, some fifteen miles to the east. Some reinforcements from Hulwan may have reached Mihran but they were not enough to prevent a further defeat and the capture of Khanaqin. It is recorded that Qaqa defeated Mihran in a personal duel, removing one of the more capable Persian commanders as an obstacle. Qaqa was now within 100 miles of Yazdgerd III’s base at Hulwan and was to appear before its walls before the end of January 638. However, upon hearing of the defeat of Mihran at Khanaqin, Yazdgerd had retreated further east into the Iranian heartland of his empire, reaching Qom, around 100 miles south of modern Tehran. This hopping from Ctesiphon to Hulwan to Qom was to become a repeating pattern for the rest of Yazdgerd’s life as he attempted to outrun the Muslim advance whilst at the same time trying to bring together an army strong enough to retake his lost lands.
With the emperor gone and only a modest garrison left to defend it, Hulwan also swiftly fell. Having settled affairs with the citizenry, the ever ambitious Qaqa then sent to his commander, Sa’d, asking if he could drive further into Iran in pursuit of the fleeing Yazdgerd. Sa’d himself appears to have been in favour of such an advance, perhaps thinking that the Persians were sure to return once they had reorganised their forces. However, Umar was unwilling to further stretch his forces given the effects of the `Year of Ashes’ and the Plague of Amwas throughout 638 and 639 and, as he had done in ordering his men to pull back from a potentially decisive confrontation in Roman Anatolia, he denied Qaqa and Sa’d permission to continue east. What is now the border between Iran and Iraq was to be the effective frontier between the lands of the caliph and those of the Persian emperor, albeit temporarily.
The loss of Mesopotamia, let alone their capital at Ctesiphon, was a huge blow, not just to the prestige of the Sassanids but perhaps more importantly to their continued ability to wage war, as those provinces contained a vast proportion of their population and tax revenue. The Persians still held significant territories all the way east to the Oxus and Indus rivers and their Roman neighbours had demonstrated that by identifying the strategic necessity of regrouping such losses were survivable. However, as will be seen, Yazdgerd and his advisers would not exhibit the same restraint and strategic good sense of Heraclius, allowing their loss of dignity to force them into challenging this `Iran-Iraq’ frontier before they had laid any defensive or infrastructural groundwork.
The Arabs appear to have known exactly how and where to fatally strike the Sassanians on the battlefield, thanks in part to the ex-Sassanian forces now fighting in their ranks.
The remainder of Yazdegird’s troops now engaged in a disorderly retreat and became ensnared in the traps originally intended for al-Hashem.The ensuing Arab pursuit resulted in 100,000 Sassanians being killed. Much booty and goods were then captured from the Sassanian camp, along with the wives and children of the Azadan nobility, who had stayed in the camps. Disputes then appear to have broken out amongst the Arab troops as to the share of the spoils, causing a temporary lull in the Arab advance.
Yazdegird himself managed to retire to Rayy, near modern day Tehran, which became the capital. General Khosrowshonum tried to retain the fortress-city of Holwan at all costs, but failed. The loss of Holwan allowed the Arabs to soon reach Mahrod, where the local dehkhan cavalry force quickly submitted to al-Hashem. By this time, morale appears to have all but collapsed, and any sense of loyalty to the House of Sassan and the magi had all but disappeared. Arab successes, combined with the fact that numbers of professional Sassanian troops had joined the Arabs since Qadisiyyah, further undermined the motivation to resist.
After the defeat at Jalula, the battered remains of the Sassanian gund were dispatched to engage the Arabs at Khuzistan and Persis. The city of Ahwaz in Khuzistan fell, allowing the Arabs to break into Persis proper. Bahrain Island in the Persian Gulf was also used as a naval base to land troops into Persis, leading to the fall of Istakhr after the battle of Tavoos. The remnants of the Sassanian forces rallied under General Shahrak and, with the support of inhabitants of Persis, put up a powerful resistance. However, the Arabs prevailed and captured Ramhormuz, Tustar, Manadir, and Shushtar. The resistance at Shushtar, one of the most important bastions of defense in southern Persia, was prolonged, bitter, and bloody. Hormuzan, the commander of the Sassanian forces in the southwest of the Empire, deployed his forces outside the city as the Arabs approached. The battle was yet another defeat for the Sassanians, and the survivors retreated into the city to prepare for a siege. However, one of the upper nobles had betrayed the city and that evening the conspirators killed the sentries and opened the gates to the Arabs. Hormuzan and what remained of his forces made their last stand in the citadel, sueing for peace only when supplies were exhausted. Persis and Khuzistan took a long time to subdue, even as the caliphate reached Spain.