Battle of Yarmuk
Cavalry armour consisted of a hauberk with a mail coif and a helmet with a pendant: a throat-guard lined with fabric and having a fringe and cheek piece. Infantry was similarly equipped with a hauberk, a helmet and leg armour. Light lamellar and scale armour was also used.

Tensions in 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 Yarmuk 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 Yarmuk, 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 Qanatir (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 Romans, Armenians and Arabs. Longstanding ecclesiastical feuds between the Monophysite and Chalcedonian factions, of negligible direct impact, certainly inflamed underlying tensions. The effect of the feuds was decreased coordination and planning, one of the reasons for the catastrophic Byzantine defeat.

Battle: The battle lines of the Muslims and the Byzantines were divided into four sections: the left wing, the left centre, the right centre and the right wing. Note that the descriptions of the Muslim and the Byzantine battle lines are exactly each other's opposite: 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, probably because Yazdegerd III's forces were not yet ready for the offensive in Iraq. Accordingly, Vahan sent Gregory and then Jabalah to negotiate, but their efforts proved futile. Before the battle, on Vahan's invitation, Khalid came to negotiate peace, with a similar end. The negotiations delayed the battles for a month.
On the other hand, Umar, whose forces at Qadisiyah were threatened with confronting the Sassanid armies, ordered Sa'd ibn Abi Waqqas to enter negotiations with the Persians and to send emissaries to Yazdegerd III and his commander Rostam Farrokhzād, apparently inviting them to Islam. That 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. The 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.
Also present were such distinct companions as Sa'id ibn Zayd, Fadl ibn Abbas, Abdul-Rahman ibn Abi Bakr (the son of Abu Bakr), Abdullah ibn Umar (the son of Umar), Aban ibn Uthman (the son of Uthman), Abdulreman ibn Khalid (the son of Khalid), Abdullah ibn Ja'far (the nephew of Ali), Ammar ibn Yasir, Miqdad ibn Aswad, Abu Dharr al-Ghifari, Malik al-Ashtar, Abu Ayyub al-Ansari, Qays ibn Sa'd, Hudhayfah ibn al-Yaman, Ubada ibn as-Samit, Hisham ibn al-A'as, Abu Huraira and Ikrimah ibn Abi Jahl. As it was a citizen army, in contrast to a mercenary army, the age of the soldiers ranged from 20 (in the case of Khalid's son) to 70 (in the case of Ammar). Three of the ten companions promised paradise by Muhammad, namely Sa'id, Zubayr and Abu Ubaidah, were present at Yarmuk.
Umar, apparently wanting to defeat the Byzantines first, used 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 Yarmuk arrived in small bands, giving the impression of a continuous stream of reinforcements to demoralize the Byzantines to compel them to attack. The same tactic would be repeated again during the Battle of Qadisiyah.

Day 1: The battle began on 15 August. 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 centre, 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 Byzantine soldiers 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, 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 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.
However, Khalid had prepared for such a contingency by placing a strong outpost line in front during the night to counter surprises, which gave the Muslims time to prepare for battle. At the centre, 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 centre remained stable, but on the wings the situation was different. Qanatir, 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, and 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. The Muslim right wing enjoyed assistance from the mobile guard but not the left wing, and the numerical advantage the Byzantines enjoyed caused the Muslim positions to be overrun, with soldiers retreating towards base camps. There, the Byzantines had broken through the corps. The testudo formation that Gregory's army had adopted moved slowly but also had a good defence. 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 morale was severely damaged.
The retreating Muslim army was met by the ferocious Arab women in the camps.