Battle of Yarmuk
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. 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, which then had been directed against the Muslims.

That 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 centre) 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, but Khalid's successful counterattacks emboldened his troops despite their being smaller in number.

Day 3: On 17 August, 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 Byzantine army decided on a less ambitious plan, and 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 manoeuvrer more freely as compared to the rugged terrain at the Muslims' left flank. The junction was to be between the Muslim right centre, its right wing held by Qanatir's Slavs, to break them apart so that they would be fought separately.
Phase 1: The battle resumed with Byzantine attacks on the Muslim right flank and right centre.
After holding off the initial attacks by the Byzantines, the Muslim right wing fell back, followed by the right centre. They were again said to have been met by their own women, 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 ibn al-Walid launched an attack with his mobile guard, along with the Muslim right flank cavalry. Khalid ibn al-Walid struck at the right flank of the Byzantines left centre, and the cavalry reserve of the Muslims right centre struck at the Byzantines left centre 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: 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.
Qanatir 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 centre again fell back.
Khalid entered the fray yet again with the mobile guard. He feared a general attack on a broad front, which he would be unable to repulse, and so 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 centre, and the infantry of the Muslim right centre attacked from the front. Under the 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 centre 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 400 members of the cavalry by attacking the Byzantine front, and 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, was mortally wounded and died later in the evening.

Day 5: During the four-day offence 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, 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.
Until 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 cavalry regiments were grouped together into one powerful mounted force with the mobile guard acting as its core. The total strength of the 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-Ruqqad 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 metres (660 ft) 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, a manoeuvrer that was to prove decisive the next day.

Day 6: On 20 August, 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 the two-pronged attack, the Byzantine left wing fell back and collapsed and fell back to the Byzantine left centre, greatly disordering it.