Battle of Yarmouk

The Battle of Yarmouk (معركة اليرموك, also spelled Yarmuk, Yarmuq or, in Greek, Hieromyax, Ἱερομύαξ, or Iermouchas, Ιερμουχάς) was a major battle between the Muslim Arab forces of the Rashidun Caliphate and the armies of the Eastern Roman Empire. The battle consisted of a series of engagements that lasted for six days in August 636, near the Yarmouk River, along what is today the border between Syria and Jordan, south-east of the Sea of Galilee. The result of the battle was a complete Muslim victory which ended Byzantine rule in Syria. The Battle of Yarmouk is regarded as one of the most decisive battles in military history, and it marked the first great wave of Islamic conquests after the death of prophet Muhammad, heralding the rapid advance of Islam into the then Christian Levant.

In order to check the Arab advance and to recover lost territory, Emperor Heraclius had sent a massive expedition to the Levant in May 636. As the Roman army approached, the Arabs tactically withdrew from Syria and regrouped all their forces at the Yarmouk plains close to Arabia where, after being reinforced, they defeated the numerically superior Byzantine army. The battle is also considered to be one of Khalid ibn al-Walid's greatest military victories. It cemented his reputation as one of the greatest tacticians and cavalry commanders in history.

Prelude
During the last Byzantine–Sassanid Wars in 610, Heraclius became the emperor of the Byzantine Empire, after overthrowing Phocas. Meanwhile the Sassanid Persians conquered Mesopotamia and in 611 they overran Syria and entered Anatolia, occupying Caesarea Mazaca. Heraclius, in 612, managed to expel the Persians from Anatolia, but was decisively defeated in 613 when he launched a major offensive in Syria against the Persians. Over the following decade the Persians were able to conquer Palestine and Egypt. Meanwhile Heraclius prepared for a counterattack and rebuilt his army. Nine years later in 622, Heraclius finally launched his offensive. After his overwhelming victories over the Persians and their allies in the Caucasus and Armenia, Heraclius, in 627, launched a winter offensive against the Persians in Mesopotamia winning a decisive victory at the Battle of Nineveh thus threatening the Persian capital city of Ctesiphon. Discredited by these series of disasters, Khosrau II was overthrown and killed in a coup led by his son Kavadh II, who at once sued for peace, agreeing to withdraw from all occupied territories of the Byzantine Empire. Heraclius restored the True Cross to Jerusalem with a majestic ceremony in 629.

Meanwhile there had been rapid political development in Arabia, where Prophet Mohammad had been preaching Islam and by 630, he had successfully united most of the Arabia under a single political authority. When the Prophet died in June 632, Abu Bakr was elected Caliph and his political successor. Troubles emerged soon after Abu Bakr's succession, when several Arab tribes openly revolted against Abu Bakr, who declared war against the rebels. In what became known as the Ridda wars (Arabic for the Wars of Apostasy, 632–33), Abu Bakr managed to unite Arabia under the central authority of the Caliph at Medina.

Once the rebels had been subdued, Abu Bakr began a war of conquest, beginning with Iraq. Sending his most brilliant general, Khalid ibn al-Walid, Iraq was conquered in a series of successful campaigns against the Sassanid Persians. Abu Bakr's confidence grew, and once Khalid established his stronghold in Iraq, Abu Bakr issued a call to arms for the invasion of Syria in February 634. The Muslim invasion of Syria was a series of carefully planned and well coordinated military operations that employed strategy instead of pure strength to deal with Byzantine defensive measures. The Muslim armies, however soon proved to be too small to handle the Byzantine response, and their commanders called for reinforcements. Khalid was sent by Abu Bakr from Iraq to Syria with reinforcements and to lead the invasion. In July 634, the Byzantines were decisively defeated at Ajnadayn. Damascus fell in September 634, followed by the Battle of Fahl where the last significant garrison of Palestine was defeated and routed.

Caliph Abu Bakr died in 634. His successor, Umar, was determined to continue the Caliphate Empire's expansion deeper into Syria. Though previous campaigns led by Khalid were successful, he was replaced by Abu Ubaidah. Having secured southern Palestine, Muslim forces now advanced up the trade route where Tiberias and Baalbek fell without much struggle and the Muslims conquered Emesa early in 636. From thereon, the Muslims continued their conquest across the Levant.

Byzantine counterattack
Having seized Emesa, the Muslims were just a march away from Aleppo, a Byzantine stronghold, and Antioch, where Heraclius resided. Seriously alarmed by the series of setbacks, Heraclius prepared for a counterattack to reacquire the lost regions. In 635 Yazdegerd III, the Emperor of Persia, sought an alliance with the Byzantine Emperor. Heraclius married off his daughter (according to traditions, his grand daughter) Manyanh to Yazdegerd III, to cement the alliance. While Heraclius prepared for a major offensive in the Levant, Yazdegerd was to mount a simultaneous counterattack in Iraq, in what was meant to be a well-coordinated effort. When Heraclius launched his offensive in May 636, Yazdegerd could not coordinate with the maneuver—probably owing to the exhausted condition of his government—and what would have been a decisive plan missed the mark. Umar Ibn Al Khattab(Caliphate Rashiduun) won a decisive victory against Heraclius at Yarmouk, and used great strategy to engage and entrap Yazdegerd. Three months later Yazdegerd lost his imperial army at the Battle of Qadisiyah in November 636, ending Sassanid control west of Persia.

Byzantine preparations began in late 635 and by May 636 Heraclius had a large force concentrated at Antioch in Northern Syria. The assembled army consisted of contingents of Byzantines, Slavs, Franks, Georgians, Armenians and Christian Arabs. This force was organized into five armies, the joint leader of which was Theodore Trithourios the Sakellarios. Vahan, an Armenian and the former garrison commander of Emesa, was made the overall field commander, and had under his command a purely Armenian army. Buccinator (Qanateer), a Slavic prince, commanded the Slavs and Jabalah ibn al-Aiham, king of the Ghassanid Arabs, commanded an exclusively Christian Arab force. The remaining contingents, all European, were placed under Gregory and Dairjan. Heraclius himself supervised the operation from Antioch. Byzantine sources mention Niketas, son of the Persian general Shahrbaraz, among the commanders, but it is not certain which army he commanded.

At that time, the Rashidun army was split into four groups: one under Amr in Palestine, one under Shurahbil in Jordan, one under Yazid in the Damascus-Caesarea region and the last one under Abu Ubaidah along with Khalid at Emesa. As the Muslim forces were geographically divided, Heraclius sought to exploit this situation and planned to attack. He did not wish to engage in a single pitched battle but rather to employ central position and fight the enemy in detail by concentrating large forces against each of the Muslim corps before they could consolidate their troops. By forcing the Muslims to retreat, or by destroying Muslim forces separately, he would fulfill his strategy of recapturing lost territory. Reinforcements were sent to Caesarea under Heraclius' son Constantine III probably to tie down Yazid's forces which were besieging the town. The Byzantine imperial army moved out from Antioch and Northern Syria sometime in the middle of June 636.

The Byzantine imperial army was to operate under the following plan:
 * Jabalah's lightly armed Christian Arabs would march to Emesa from Aleppo via Hama and hold the main Muslim army at Emesa.
 * Dairjan would make a flanking movement – moving between the coast and Aleppo's road – and approach Emesa from the west, striking at the Muslims' left flank while they were being held frontally by Jabalah.
 * Gregory would strike the Muslims' right flank, approaching Emesa from the northeast via Mesopotamia.
 * Qanateer would march along the coastal route and occupy Beirut, from where he was to attack weakly defended Damascus from the west to cut off the main Muslim army at Emesa.
 * Vahan's corps would act as a reserve and would approach Emesa via Hama.

Muslim strategy
The Muslims discovered Heraclius' preparations at Shaizar through Roman prisoners. Alert to the possibility of being caught with separated forces that could be destroyed, Khalid called for a council of war. There he advised Abu Ubaidah to pull the troops back from Palestine and from Northern and Central Syria, and then to concentrate the entire Rashidun army in one place. Abu Ubaidah ordered the concentration of troops in the vast plain near Jabiya, as control of the area made cavalry charges possible and facilitated the arrival of reinforcements from Umar so that a strong, united force could be fielded against the Byzantine armies. The position also benefited from close proximity to the Rashidun stronghold of Najd, in case of retreat. Instructions were also issued to return the jizya (tribute) to the people who had paid it. However, once concentrated at Jabiya, the Muslims were subject to raids from pro-Byzantine Ghassanid forces. Encamping in the region was also precarious as a strong Byzantine force was garrisoned in Caeseara and could attack the Muslim rear while they were held in front by the Byzantine army. On Khalid's advice the Muslim forces retreated to Dara’ah (or Dara) and Dayr Ayyub, covering the gap between the Yarmouk Gorges and the Harra lava plains, and established a line of camps in the eastern part of the plain of Yarmouk. This was a strong defensive position and these maneuvers pitted the Muslims and Byzantines into a decisive battle, one which the latter had tried to avoid. During these maneuvers, there were no engagements save for a minor skirmish between Khalid's elite light cavalry and the Byzantine advance guard.

Battlefield
The battlefield lies about 65 km south-east of the Golan Heights, an upland region currently on the frontier between Israel, Jordan and Syria, east of the Sea of Galilee. The battle was fought on the plain of Yarmouk, which was enclosed on its western edges by a deep ravine known as Wadi-ur-Raqad, around 200 m deep. This ravine joins the Yarmouk River, a tributary of the Jordan River, on its south. The stream had very steep banks, ranging from 30 m–200 m in height. On the north is the Jabiya road and to the east are the Azra hills, although these hills were outside the actual field of battle. Strategically there was only one prominence in the battlefield: a 100 m elevation known as Tel al Jumm'a (Arabic for hill of gathering), and for the Muslim troops concentrated there, the hill gave a good view of the plain of Yarmouk. The ravine on the west of the battlefield was accessible at a few places in 636 AD, and had one main crossing: a bridge near the present-day village of Kafir-ul-Ma. Logistically, the Yarmouk plain had enough water supplies and pastures to sustain both armies. The plain was excellent for cavalry maneuvers.

Troop deployment
Most early Muslim accounts place the size of the Muslim forces between 24,000 and 40,000 and the number of Byzantine forces between 100,000 and 400,000. Modern estimates of the sizes of the respective armies vary: estimates for the Byzantine army are mostly between 80,000 and 150,000, with some estimates as low as 50,000 and 15,000–20,000. Estimates for the Rashidun army are between 25,000 and 40,000. These figures come from studying the logistical capabilities of the combatants, the sustainability of their respective bases of operations, and the overall manpower constraints affecting the Romans and Arabs. Most scholars, however, agree that the Byzantine army and their allies outnumbered the Muslim Arabs by a sizeable margin.

The Rashidun army
During a council of war, the command of the Muslim army was transferred to Khalid by Abu Ubaidah, Commander in Chief of the Muslim army. After taking command, Khalid reorganized the army into 36 infantry regiments and four cavalry regiments, with his cavalry elite, the mobile guard, held in reserve. The army was organized in the Tabi'a formation; a tight, defensive infantry formation. The army was lined up on a front of 12 km, facing west, with its left flank lying south on the Yarmouk River a mile before the ravines of Wadi al Allan began. The army's right flank was on the Jabiya road in the north across the heights of Tel al Jumm'a, with substantial gaps between the divisions so that their frontage would match that of the Byzantine battle line at 13 km. The center of the army was under the command of Abu Ubaidah ibn al-Jarrah (left center) and Shurahbil bin Hasana (right center). The left wing was under the command of Yazid and the right wing was under Amr ibn al-A'as. Center, left and right wings were given cavalry regiments, to be used as a reserve for counter-attack in case they were pushed back by the Byzantines. Behind the center stood the mobile guard under the personal command of Khalid. If and when Khalid was too occupied in leading the general army, Dharar ibn al-Azwar would command the mobile guard. Over the course of the battle, Khalid would repeatedly make critical and decisive use of this mounted reserve. Khalid sent out several scouts to keep the Byzantines under observation. In late July 636, Vahan sent Jabalah with his lightly armored Christian Arab forces to reconnoiter-in-force, but they were repulsed by the mobile guard. After this skirmish, no engagement occurred for a month.

Weaponry
Helmets used included gilded helmets similar to the silver helmets of the Sassanid empire. Mail was commonly used to protect the face, neck and cheeks either as an aventail from the helmet or as a mail coif. Heavy leather sandals as well as Roman-type sandal boots were also typical of the early Muslim soldiers. Armor included hardened leather scale or lamellar armor and mail armor. Infantry soldiers were more heavily armored than horsemen. Large wooden or wickerwork shields were used. Long-shafted spears were used, with infantry spears being 2.5 m long and cavalry spears being up to 5.5 m long. Short infantry swords like the Roman gladius and Sassanid long swords were used; long swords were usually carried by horsemen. Swords were hung in baldrics. Bows were about 2 m long when unbraced, similar in size to the famous English longbow. The maximum useful range of the traditional Arabian bow was about 150 m. Early Muslim archers, while being infantry archers without the mobility of horseback archer regiments, proved to be very effective in defending against light and unarmored cavalry attacks.

The Byzantine army
A few days after the Muslims encamped at the Yarmouk plain, the Byzantine army, preceded by the lightly armed Ghassanids of Jabalah, moved forward and established strongly fortified camps just north of the Wadi-ar-Raqqad. The right flank of the Byzantine army was at the south end of the plains, near the Yarmouk River and about a mile before the ravines of Wadi al Allan began. The left flank of the Byzantines was at the north, a short distance before the Hills of Jabiya began, and was relatively exposed. Vahan deployed the Imperial Army facing east, with a front about 13 km long, as he was trying to cover the whole area between the Yarmouk gorge in the south and the Roman road to Egypt in the north, and substantial gaps had been left between the Byzantine divisions. The right wing was commanded by Gregory and the left by Qanateer. The center was formed by the army of Dairjan and the Armenian army of Vahan, both under the overall command of Dairjan. The Roman regular heavy cavalry, the cataphract, was distributed equally among the four armies, each army deploying its infantry at the forefront and its cavalry as a reserve in the rear. Vahan deployed Jabalah's Christian Arabs, mounted on horses and camels, as a skirmishing force, screening the main army until its arrival. Early Muslim sources mention that the army of Gregory had used chains to link together its foot-soldiers, who had all taken an oath of death. The chains were in 10-man lengths and were used as a proof of unshakeable courage on the part of the men, who thus displayed their willingness to die where they stood and never retreat. The chains also acted as an insurance against a breakthrough by enemy cavalry. However, modern historians suggest that the Byzantines adopted the Graeco-Roman testudo military formation, in which soldiers would stand shoulder-to-shoulder with shields held high and an arrangement of 10 to 20 men would be completely shielded on all sides from missile fire, each soldier providing cover for an adjoining companion.

Weaponry
The Byzantine cavalry was armed with a long sword, known as the spathion. They would also have had a light wooden lance, known as a kontarion and a bow (toxarion) with forty arrows in a quiver, hung from a saddle or from the belt. Heavy infantry, known as skoutatoi, had a short sword and a short spear. The lightly armed Byzantine troops and the archers carried a small shield, a bow hung from the shoulder across the back and a quiver of arrows. Cavalry armor consisted of a hauberk with a mail coif and a helmet with a pendant, i.e. a throat-guard lined with fabric and having a fringe and cheek piece. Infantry was similarly equipped with a hauberk, a helmet and leg armor. Light lamellar and scale armor was also used.

Tensions in the Byzantine army
Khalid's strategy of withdrawing from the occupied areas and concentrating all of his troops for a decisive battle forced the Byzantines to concentrate their five armies in response. The Byzantines had for centuries avoided engaging in large-scale decisive battles, and the concentration of their forces created logistical strains for which the empire was ill-prepared. Damascus was the closest logistical base, but Mansur, leader of Damascus, could not fully supply the massive Byzantine army that was gathered at the Yarmouk plain. Several clashes were reported with local citizens over supply requisition, as summer was at an end and there was a decline of pasturage. Greek court sources accused Vahan of treason for his disobedience to Heraclius' command not to engage in large-scale battle with Arabs. Given the massing of the Muslim armies at Yarmouk, however, Vahan had little choice but to respond in kind. Relations between the various Byzantine commanders were also fraught with tension. There was a struggle for power between Trithurios and Vahan, Jarajis, and Qanateer (Buccinator). Jabalah, the Christian Arab leader, was largely ignored, to the detriment of the Byzantines given his knowledge of the local terrain. An atmosphere of mistrust thus existed between the Greeks, Armenians, and Arabs. Longstanding ecclesiastical feuds between the Monophysite and Chalcedonian factions, while of negligible direct impact, certainly inflamed underlying tensions. The effect of these feuds was decreased coordination and planning, one of the reasons for the catastrophic Byzantine defeat.

Battle
For a good understanding of the description of the battle, it is useful to be acquainted with the divisions of opposing forces. The battle lines of the Muslims and the Byzantines were divided into four sections: the left wing, the left center, the right center and the right wing. Note that the descriptions of the Muslim and the Byzantine battle lines are exactly each other's opposite, i.e.: so the Muslim right wing faced the Byzantine left wing (see image).

Vahan was instructed by Heraclius not to engage in battle until all avenues of diplomacy had been explored. This was probably because Yazdegerd III's forces were not yet ready for the offensive in Iraq. Accordingly, Vahan sent Gregory and then Jabalah to negotiate, though their efforts proved futile. Before the battle, on Vahan's invitation, Khalid came to negotiate peace, to a similar end. These negotiations delayed the battles for a month. On the other hand, Caliph Umar, whose forces at Qadisiyah were threatened with confronting the Sassanid armies, ordered Sa`d ibn Abi Waqqas to enter into negotiations with the Persians and send emissaries to Yazdegerd III and his commander Rostam Farrokhzād, apparently inviting them to Islam. This was most probably the delaying tactic employed by Umar on the Persian front. Meanwhile he sent reinforcements of 6,000 troops, mostly from Yemen, to Khalid. This force included 1,000 Sahaba (companions of Muhammad), among whom were 100 veterans of the Battle of Badr, the first battle in Islamic history, and included citizens of the highest rank, such as Zubayr ibn al-Awwam, Abu Sufyan, and his wife Hind bint Utbah.

Umar, apparently wanting to defeat the Byzantines first, employed the best Muslim troops against them. The continuing stream of Muslim reinforcements worried the Byzantines, who fearing that the Muslims with such reinforcements would grow powerful, decided that they had no choice but to attack. The reinforcements that were sent to the Muslims at Yarmouk arrived in small bands, giving the impression of a continuous stream of reinforcements, in order to demoralize the Byzantines and compel them to attack. The same tactic would be repeated again during the Battle of Qadisiyah.

Day 1
The battle began on 15 August 636. At dawn both armies lined up for battle less than a mile apart. It is recorded in Muslim chronicles that before the battle started, George, a unit commander in the Byzantine right center, rode up to the Muslim line and converted to Islam; he would die the same day fighting on the Muslim side. The battle began as the Byzantine army sent its champions to duel with the Muslim mubarizun. The mubarizun were specially trained swordsmen and lancers, with the objective to slay as many enemy commanders as possible to damage their morale. At midday, after losing a number of commanders in the duels, Vahan ordered a limited attack with a third of his infantry forces to test the strength and strategy of the Muslim army and, using their overwhelming numerical and weaponry superiority, achieve a breakthrough wherever the Muslim battle line was weak. However the Byzantine assault lacked determination; many soldiers of the Imperial Army were unable to press the attack against the Muslim veterans. The fighting was generally moderate, although in some places it was especially intense. Vahan did not reinforce his forward infantry two-thirds of which was kept in reserve with one-third deployed to engage the Muslims, and at sunset both armies broke contact and returned to their respective camps.

Day 2
Phase 1: On 16 August 636, Vahan decided in a council of war to launch his attack just before dawn, to catch the Muslim force unprepared as they conducted their morning prayers. He planned to engage his two central armies with the Muslim centre in an effort to stall them while the main thrusts would be against the wings of the Muslim army, which would then either be driven away from the battlefield or pushed towards the centre. To observe the battlefield, Vahan had a large pavilion built behind his right wing with an Armenian bodyguard force. He ordered the army to prepare for the surprise attack. Unbeknownst to the Byzantines, Khalid had prepared for such a contingency by placed a strong outpost line in front during the night to counter surprises, which gave the Muslims time to prepare for battle. At the center, the Byzantines did not press hard, intending to pin down the Muslim centre corps in their position and preventing them from aiding the Muslim army in other areas. Thus the center remained stable. But on the wings the situation was different. Qanateer, commanding the Byzantine left flank which consisted of mainly Slavs, attacked in force, and the Muslim infantry on the right flank had to retreat. Amr, the Muslim right wing commander ordered his cavalry regiment to counterattack, which neutralized the Byzantine advance and stabilized the battle line on the right for some time, but the Byzantine numerical superiority caused them to retreat towards the Muslim base camp.

Phase 2: Khalid, aware of the situation at the wings, ordered the cavalry of the right wing to attack the northern flank of the Byzantine left wing while he with his mobile guard attacked the southern flank of the Byzantine left wing, while the Muslim right wing infantry attacked from the front. The three-pronged attack forced the Byzantine left wing to abandon the Muslim positions they had gained on, and Amr regained his lost ground and started reorganizing his corps for another round. The situation on the Muslim left wing which Yazid commanded was considerably more serious. Whilst the Muslim right wing enjoyed assistance from the mobile guard, the left wing did not and the numerical advantage the Byzantines enjoyed caused the Muslim positions to be overrun, with soldiers retreating towards base camps. Here the Byzantines had broken through the corps. The testudo formation that Gregory's army had adopted moved slowly but also had a good defense. Yazid used his cavalry regiment to counterattack but was repulsed. Despite stiff resistance, the warriors of Yazid on the left flank finally fell back to their camps and for a moment Vahan's plan appeared to be succeeding. The centre of the Muslim army was pinned down and its flanks had been pushed back. However, neither flank had broken, though their morale was severely damaged. The retreating Muslim army was met by the ferocious Arab women in the camps. Led by Hind, the Muslim women dismantled their tents and armed with tent poles charged at their husbands and fellow men singing an improvised song from the Battle of Uhud that then had been directed against the Muslims.

''O you who run from a constant woman

Who has both beauty and virtue;

And leave her to the infidel,

The hated and evil infidel,

To possess, disgrace and ruin.'' This boiled the blood of the retreating Muslims so much that they returned to the battlefield.

Phase 3: After managing to stabilize the position on the right flank, Khalid ordered the mobile guard cavalry to provide relief to the battered left flank. Khalid detached one regiment under Dharar ibn al-Azwar and ordered him to attack the front of the army of Dairjan (left center) in order to create a diversion and threaten the withdrawal of the Byzantine right wing from its advanced position. With the rest of the cavalry reserve he attacked Gregory's flank. Here again, under simultaneous attacks from the front and flanks, the Byzantines fell back, but more slowly because they had to maintain their formation. At sunset the central armies broke contact and withdrew to their original positions and both fronts were restored along the lines occupied in the morning. The death of Dairjan and the failure of Vahan's battle plan left the larger Imperial army relatively demoralized, whereas Khalid's successful counterattacks emboldened his troops despite their being smaller in number.

Day 3
On 17 August 636, Vahan pondered over his failures and mistakes of the previous day, where he launched attacks against respective Muslim flanks, but after initial success, his men were pushed back. What bothered him the most was the loss of one of his commanders. The imperial Byzantine army decided on a less ambitious plan, Vahan now aimed to break the Muslim army at specific points. He decided to press upon the relatively exposed right flank, where his mounted troops could maneuver more freely as compared to the rugged terrain at the Muslims' left flank. And it was decided to charge at the junction between the Muslim right center and its right wing held by Qanateer's Slavs, to break the two apart and to fight them separately.

Phase 1: The battle resumed with Byzantine attacks on the Muslim right flank and right center. After holding off the initial attacks by the Byzantines, the Muslim right wing fell back, followed by the right center. They were again said to have met by their own womenfolk who abused and shamed them. The corps, however, managed to reorganize some distance from the camp and held their ground preparing for a counterattack.

Phase 2: Knowing that the Byzantine army was focusing on the Muslim right, Khalid launched an attack with his mobile guard, along with the Muslim right flank cavalry. Khalid struck at the right flank of the Byzantines left center, and the cavalry reserve of the Muslims right center struck at the Byzantines left center at its left flank. Meanwhile he ordered the Muslims' right wing cavalry to strike at the left flank of the Byzantines left wing. The combat soon developed into a bloodbath. Many fell on both sides. Khalid's timely flanking attacks again saved the day for Muslims and by dusk the Byzantines had been pushed back to the positions they had at the start of the battle.

Day 4
18 August 636, the fourth day, was to prove decisive. Phase 1: Vahan decided to persist with the previous day's war plan as he had been successful in inflicting damage on the Muslim right. Qanateer led two armies of Slavs against the Muslim right wing and right centre with some assistance from the Armenians and Christian Arabs led by Jabalah. The Muslim right wing and right center again fell back. Khalid entered the fray yet again with this mobile guard. He feared a general attack on a broad front which would he wouldn't be able to repulse and as a precaution ordered Abu Ubaidah and Yazid on the left centre and the left wings respectively to attack the Byzantine armies at the respective fronts. The attack would result in stalling the Byzantine front and prevent a general advance of the Imperial army.

Phase 2: Khalid divided his mobile guard into two divisions and attacked the flanks of the Byzantine left center, while the infantry of the Muslim right center attacked from front. Under this three-pronged flanking manoeuvre, the Byzantines fell back. Meanwhile the Muslim right wing renewed its offense with its infantry attacking from the front and the cavalry reserve attacking the northern flank of the Byzantine left wing. As the Byzantine left center retreated under three-pronged attacks of Khalid, the Byzantine left wing, having been exposed at its southern flank, also fell back.

While Khalid and his mobile guard were dealing with the Armenian front throughout the afternoon, the situation on the other end was worsening. Byzantine horse-archers had taken to the field and subjected Abu Ubaidah and Yazid's troops to intense archery preventing them from penetrating their Byzantine lines. Many Muslim soldiers lost their sight to Byzantine arrows on that day, which thereafter became known as the "Day of Lost Eyes". The veteran Abu Sufyan is also believed to have lost an eye that day. The Muslim armies fell back except for one regiment led by Ikrimah bin Abi Jahal, which was on the left of Abu Ubaidah's corps. Ikrimah covered the retreat of the Muslims with his four hundred cavalry by attacking the Byzantine front, while the other armies reorganized themselves to counterattack and regain their lost positions. All of Ikrimah's men were either seriously injured or dead that day. Ikrimah, a childhood friend of Khalid's was mortally wounded and died later in the evening.

Day 5
During the four day offense of Vahan, his troops had failed to achieve any breakthrough and had suffered heavy casualties, especially during the mobile guard's flanking counterattacks. Early on 19 August 636, the fifth day of the battle, Vahan sent an emissary to the Muslim camp for a truce for the next few days so that fresh negotiations could be held. He supposedly wanted time to reorganize his demoralized troops. But Khalid deemed victory to be in reach and he declined the offer. Up till now, the Muslim army had adopted a largely defensive strategy, but knowing that the Byzantines were apparently no longer eager for battle, Khalid now decided to take the offensive and reorganized his troops accordingly. All the cavalry regiments were grouped together into one powerful mounted force with the mobile guard acting as its core. The total strength of this cavalry group was now about 8,000 mounted warriors, an effective mounted corps for an offensive attack the next day. The rest of the day passed uneventfully. Khalid planned to trap Byzantine troops, cutting off their every route of escape. There were three natural barriers, the three gorges in the battlefield with their steep ravines, Wadi-ur-Raqqad at west, Wadi al Yarmouk in south and Wadi al Allah in east. The northern route was to be blocked by Muslim cavalry. There were however, some passages across the 200 m deep ravines of Wadi-ur-Raqqad in west, strategically the most important one was at Ayn al Dhakar, a bridge. Khalid sent Dharar with 500 cavalry at night to secure that bridge. Dharar moved around the northern flank of Byzantines and captured the bridge. This maneuver was to prove decisive the next day.

Day 6
On 20 August 636, the final day of the battle, Khalid put into action a simple but bold plan of attack. With his massed cavalry force he intended to drive the Byzantine cavalry entirely off the battlefield so that the infantry, which formed the bulk of the imperial army, would be left without cavalry support and thus would be exposed when attacked from the flanks and rear. At the same time he planned to push a determined attack to turn the left flank of the Byzantine army and drive them towards the ravine to the west.

Phase 1: Khalid ordered a general attack on the Byzantine front and galloped his cavalry around the left wing of the Byzantines. Part of his cavalry engaged the Byzantine left wing cavalry while the rest of it attacked the rear of the Byzantine left wing infantry. Meanwhile the Muslim right wing pressed against it from the front. Under this two-pronged attack, the Byzantine left wing fell back and collapsed and fell back to the Byzantine left center, greatly disordering it. The remaining Muslim cavalry then attacked the Byzantine left wing cavalry at the rear while they were held frontally by the other half of the Muslim cavalry, routing them off the battlefield to the north. The Muslim right wing infantry now attacked the Byzantine left center at its left flank while the Muslim right center attacked from front.

Phase 2: Vahan, noticing the huge cavalry maneuver of the Muslims, ordered his cavalry to group together, but was not quick enough; before Vahan could organize his disparate heavy cavalry squadrons, Khalid had wheeled his cavalry back to attack the concentrating Byzantine cavalry squadrons, falling upon them from the front and the flank while they were still moving into formation. The disorganized and disoriented Byzantine heavy cavalry was soon routed and dispersed to the north, leaving the infantry to its fate.

Phase 3: With the Byzantine cavalry completely routed, Khalid turned to the Byzantine left center which already held the two-pronged attack of the Muslim infantry. The Byzantine left center was attacked at its rear by Khalid's cavalry and was finally broken.

The last phase: With the retreat of the Byzantine left center, a general Byzantine retreat started. Khalid took his cavalry north to block the northern route of escape. The Byzantines retreated west towards Wadi-ur-Raqqad where there was a bridge at Ayn al Dhakar for safe crossing across the deep gorges of the ravines of Wadi-ur-Raqqad. Dharar had already captured the bridge as part of Khalid's plan the night before. A unit of 500 mounted troops had been sent to block this passageway. In fact, this was the route by which Khalid wanted the Byzantines to retreat all along. The Byzantines were surrounded from all sides now. Some fell into the deep ravines off the steep slopes, others tried to escape in the waters, only to be smashed on the rocks below and again others were killed in their flight. Nevertheless a large number of the soldiers managed to escape the slaughter. Jonah, the Greek informant of the Rashidun army during the Conquest of Damascus died in this battle. The Muslims took no prisoners in this battle, although they may have captured some during the subsequent pursuit. Theodore Trithurios died on the battlefield, while Niketas managed to escape and reach Emesa. Jabalah ibn al-Ayham also managed to escape and later, for a short time, came to terms with the Muslims, but soon defected to the Byzantine court again.

Aftermath
Immediately after this operation was over, Khalid and his mobile guard moved north to pursue the retreating Byzantine soldiers; he found them near Damascus and attacked. In the ensuing fight the commander-in-chief of the imperial army, the Armenian prince Vahan who had escaped the fate of most of his men at Yarmouk, was killed. Khalid then entered Damascus where he was said to have been welcomed by the local residents, thus recapturing the city.

When news of the disaster reached the Byzantine Emperor Heraclius at Antioch, he was devastated and enraged. He blamed his wrongdoings for the loss, primarily referring to his incestuous marriage to his niece Martina. He would have tried to reconquer the province if he had the resources, but now he had neither the men nor the money to defend the province any more. Rather he retreated to the cathedral of Antioch where he observed a solemn service of intercession. He summoned a meeting of his advisers at the cathedral and scrutinized the situation. He was told almost unanimously and accepted the fact that the defeat was God's decision and a result of the sins of the people of the land, including him. Heraclius took to the sea on a ship to Constantinople in the night. It is said that as his ship was set sail, he bade a last farewell to Syria, saying:

''Farewell, a long farewell to Syria, my fair province. Thou art an infidel's (enemy's) now. Peace be with you, O Syria – what a beautiful land you will be for the enemy''.

Haraclius abandoned Syria with the holy relic of the True Cross which was, along with other relics held at Jerusalem, secretly boarded on ship by Parthia of Jerusalem, just to protect it from the invading Arabs. It is said that the emperor had a fear of water. and a pontoon bridge was made for Heraclius to cross the Bosphorus to Constantinople. After abandoning Syria, the Emperor began to concentrate on his remaining forces for the defence of Anatolia and Egypt instead. Byzantine Armenia fell to the Muslims in 638–39 after which Heraclius created a buffer zone in central Anatolia by ordering all the forts east of Tarsus to be evacuated. In 639–642 Muslims invaded and captured Byzantine Egypt, led by Amr ibn al-A'as – who had commanded the right flank of the Rashidun army at Yarmouk.

Evaluation
The Battle of Yarmouk can be seen as an example in military history where an inferior force manages to overcome a superior force by superior generalship.

The Imperial Byzantine commanders allowed their enemy to have the battlefield of his choosing. Even then they were at no substantial tactical disadvantage. Khalid knew all along that he was up against a force superior in numbers and, until the last day of the battle, he conducted an essentially defensive campaign suited to his relatively limited resources. When he decided to take the offensive and attack on the final day of battle, he did so with a degree of imagination, foresight and courage that none of the Byzantine commanders managed to display. Although he commanded a numerically inferior force and needed all the men he could muster, he nevertheless had the confidence and foresight to dispatch a cavalry regiment the night before his assault to seal off a critical path of the retreat he anticipated for the enemy army.

Khalid ibn al-Walid was one of the finest cavalry commanders in history and his use of mounted warriors throughout the battle showed just how well he understood the potential strengths and weaknesses of his mounted troops. His mobile guard moved quickly from one point to another, always changing the course of events wherever they appeared, and then just as quickly galloping away to change the course of events elsewhere on the field.

Vahan and his Byzantine commanders did not manage to deal with this mounted force and use the sizable advantage of their army effectively. Their own Byzantine cavalry never played a significant role in the battle and were held in static reserve for most of the six days. They never pushed their attacks and even when they obtained what could have been a decisive breakthrough on the fourth day, they were unable to exploit it. There appeared to be a decided lack of resolve among the Imperial commanders, though this may have been caused by difficulties commanding the army because of internal conflict. Moreover, many of the Arab auxiliaries were mere levies, while the Muslim Arab army consisted for a much larger part of veteran troops.

The original strategy of Heraclius, to destroy the Muslim troops in Syria, needed a rapid and quick deployment, but the commanders on the ground never displayed these qualities. Ironically, on the field at Yarmouk, Khalid carried out on a small tactical scale what Heraclius had planned on a grand strategic scale: by rapidly deploying and manoeuvering his forces, Khalid was able to temporarily concentrate sufficient forces at specific locations on the field to defeat the larger Byzantine army in detail. Vahan was never able to make his numerical superiority count, perhaps because of the unfavorable terrain that prevented large-scale deployment. However, at no point did Vahan attempt to concentrate a superior force to achieve a critical breakthrough. Although he was on the offensive 5 days out of the six, his battle line remained remarkably static. This all stands in stark contrast to the very successful offensive plan that Khalid carried out on the final day, when he reorganised virtually all his cavalry and committed them to a grand manoeuvre that won the battle. George F. Nafziger, in his book Islam at war, describes the battle as: Although Yarmouk is little known today, it is one of the most decisive battles in human history...... Had Heraclius' forces prevailed, the modern world would be so changed as to be unrecognizable.