Khalid ibn al-Walid
Several traditions relate the Muslims' capture of Damascus. The most popular narrative is preserved by the Damascus-based Ibn Asakir (d. 1175), according to whom Khalid and his men breached the Bab Sharqi gate. Khalid and his men scaled the city's eastern walls and killed the guards and other defenders at Bab Sharqi. As his forces entered from the east, Muslim forces led by Abu Ubayda had entered peacefully from the western Bab al-Jabiya gate after negotiations with Damascene notables led by Mansur ibn Sarjun, a high-ranking city official. The Muslim armies met up in the city center where capitulation terms were agreed. On the other hand, al-Baladhuri holds that Khalid entered peacefully from Bab Sharqi while Abu Ubayda entered from the west by force. Modern research questions Abu Ubayda's arrival in Syria by the time of the siege. Caetani cast doubt about the aforementioned traditions, while the orientalist Henri Lammens substituted Abu Ubayda with Yazid ibn Abi Sufyan.
In the versions of the Syriac author Dionysius of Tel Mahre (d. 845) and the Melkite patriarch Eutychius of Alexandria (d. 940), the Damascenes led by Mansur, having become weary of the siege and convinced of the besiegers' determination, approached Khalid at Bab Sharqi with an offer to open the gate in return for assurances of safety. Khalid accepted and ordered the drafting of a capitulation agreement. Although several versions of Khalid's treaty were recorded in the early Muslim and Christian sources, they generally concur that the inhabitants' lives, properties and churches were to be safeguarded, in return for their payment of the jizya (poll tax). Imperial properties were confiscated by the Muslims. The treaty probably served as the model for the capitulation agreements made throughout Syria, as well Iraq and Egypt, during the early Muslim conquests.
Although the accounts cited by al-Waqidi (d. 823) and Ibn Ishaq agree that Damascus surrendered in August/September 635, they provide varying timelines of the siege ranging from four to fourteen months.

Battle of Yarmouk: In the spring of 636, Khalid withdrew his forces from Damascus to the old Ghassanid capital at Jabiya in the Golan. He was prompted by the approach of a large Byzantine army dispatched by Heraclius, consisting of imperial troops led by Vahan and Theodore Trithyrius and frontier troops, including Christian Arab light cavalry led by the Ghassanid phylarch Jabala ibn al-Ayham and Armenian auxiliaries led by a certain Georgius (called Jaraja by the Arabs). The sizes of the forces are disputed by modern historians; Donner holds the Byzantines outnumbered the Muslims four to one, Walter E. Kaegi writes the Byzantines "probably enjoyed numerical superiority" with 15,000–20,000 or more troops, and John Walter Jandora holds there was likely "near parity in numbers" between the two sides with the Muslims at 36,000 men (including 10,000 from Khalid's army) and the Byzantines at about 40,000.
The Byzantine army set up camp at the Ruqqad tributary west of the Muslims' positions at Jabiya. Khalid consequently withdrew, taking up position north of the Yarmouk River, close to where the Ruqqad meets the Yarmouk. The area spanned high hilltops, water sources, critical routes connecting Damascus to the Galilee and historic pastures of the Ghassanids. For over a month, the Muslims held the strategic high ground between Adhri'at (modern Daraa) and their camp near Dayr Ayyub and bested the Byzantines in a skirmish outside Jabiya on 23 July 636. Jandora asserts that the Byzantines' Christian Arab and Armenian auxiliaries deserted or defected, but that the Byzantine force remained "formidable", consisting of a vanguard of heavy cavalry and a rear guard of infantrymen when they approached the Muslim defensive lines.

Khalid split his cavalry into two main groups, each positioned behind the Muslims' right and left infantry wings to protect his forces from a potential envelopment by the Byzantine heavy cavalry. He stationed an elite squadron of 200–300 horsemen to support the center of his defensive line and left archers posted in the Muslims' camp near Dayr Ayyub, where they could be most effective against an incoming Byzantine force. The Byzantines' initial assaults against the Muslims' right and left flanks successively failed, but they kept up the momentum until the entire Muslim line fell back or, as contemporary Christian sources maintain, feigned retreat.
The Byzantines pursued the Muslims into their camp, where the Muslims had their camel herds hobbled to form a series of defensive perimeters from which the infantry could fight and which Byzantine cavalries could not easily penetrate. As a result, the Byzantines were left vulnerable to attack by Muslim archers, their momentum was halted and their left flank exposed. Khalid and his cavalries used the opportunity to pierce the Byzantines' left flank, taking advantage of the gap between the Byzantine infantry and cavalry. Khalid enveloped the opposing heavy cavalry on either side, but intentionally left an opening from which the Byzantines could only escape northward, far from their infantry. According to the 9th-century Byzantine historian Theophanes, the Byzantine infantry mutinied under Vahan, possibly in light of Theodore's failure to counter the attack on the cavalry. The infantry was subsequently routed.
The Byzantine cavalry, meanwhile, had withdrawn north to the area between the Ruqqad and Allan tributaries. Khalid sent a force to pursue and prevent them from regrouping. He followed up with a nighttime operation in which he seized the Ruqqad bridge, the only viable withdrawal route for the Byzantines. The Muslims then assaulted the Byzantines' camps on 20 August and massacred most of the Byzantine troops, or induced panic in Byzantine ranks, causing thousands to die in the Yarmouk's ravines in an attempt to make a westward retreat.
Jandora credits the Muslim victory at Yarmouk to the cohesion and "superior leadership" of the Muslim army, particularly the "ingenuity" of Khalid, in comparison to the widespread discord in the Byzantine army's ranks and the conventional tactics of Theodorus, which Khalid "correctly anticipated". In Gil's view, Khalid's withdrawal before the army of Heraclius, the evacuation of Damascus and the counter-movement on the Yarmouk tributaries "are evidence of his excellent organising ability and his skill at manoeuvring on the battlefield". The Byzantine rout marked the destruction of their last effective army in Syria, immediately securing earlier Muslim gains in Palestine and Transjordan and paving the way for the recapture of Damascus in December, this time by Abu Ubayda, and the conquest of the Beqaa Valley and ultimately the rest of Syria to the north. In Jandora's assessment, Yarmouk was one of "the most important battles of World History", ultimately leading to Muslim victories which expanded the Caliphate between the Pyrenees mountains and Central Asia.

Demotion: Khalid was retained as supreme commander of the Muslim forces in Syria between six months and two years from the start of Umar's caliphate, depending on the source. Modern historians mostly agree that Umar's dismissal of Khalid probably occurred in the aftermath of Yarmouk. The caliph appointed Abu Ubayda to Khalid's place, reassigned his troops to the remaining Muslim commanders and subordinated Khalid under the command of one of Abu Ubayda's lieutenants; a later order deployed the bulk of Khalid's former troops to Iraq. Varied causes for Khalid's dismissal from the supreme command are cited by the early Islamic sources. Among them were his independent decision-making and minimal coordination with the leadership in Medina; older allegations of moral misconduct, including his execution of Malik ibn Nuwayra and subsequent marriage to Malik's widow; accusations of generous distribution of booty to members of the tribal nobility to the detriment of eligible early Muslim converts; personal animosity between Khalid and Umar; and Umar's uneasiness over Khalid's heroic reputation among the Muslims, which he feared could develop into a personality cult.
The modern historians De Goeje, William Muir and Andreas Stratos viewed Umar's enmity with Khalid as a contributing cause of Khalid's dismissal. Shaban acknowledges the enmity but asserts it had no bearing on the caliph's decision. De Goeje dismisses Khalid's extravagant grants to the tribal nobility, a common practice among the early Muslim leaders including Muhammad, as a cause for his sacking. Muir, Becker, Stratos and Philip K. Hitti have proposed that Khalid was ultimately dismissed because the Muslim gains in Syria in the aftermath of Yarmouk required the replacement of a military commander at the helm with a capable administrator such as Abu Ubayda.
Athamina doubts all the aforementioned reasons, arguing the cause "must have been vital" at a time when large parts of Syria remained under Byzantine control and Heraclius had not abandoned the province.