Khalid ibn al-Walid
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. Athamina holds that "with all his military limitations", Abu Ubayda would not have been considered "a worthy replacement for Khālid's incomparable talents". Medina's lack of a regular standing army, the need to redeploy fighters to other fronts, and the Byzantine threat to Muslim gains in Syria all required the establishment of a defense structure based on the older-established Arab tribes in Syria, which had served as confederates of Byzantium. After Medina's entreaties to the leading confederates, the Ghassanids, were rebuffed, relations were established with the Kalb, Judham and Lakhm. These tribes likely considered the large numbers of outside Arab tribesmen in Khalid's army as a threat to their political and economic power. Khalid's initial force of 500–800 men had swelled to as high as 10,000 as a result of tribesmen joining his army's ranks from the Iraqi front or Arabia and as high as 30,000–40,000 factoring in their families. Athamina concludes Umar dismissed Khalid and recalled his troops from Syria as an overture to the Kalb and their allies.

Operations in northern Syria: Abu Ubayda and Khalid proceeded from Damascus northward to Homs (called Emesa by the Byzantines) and besieged the city probably in the winter of 636–637. The siege held amid a number of sorties by the Byzantine defenders and the city capitulated in the spring. Per the surrender terms, taxes were imposed on the inhabitants in return for guarantees of protection for their property, churches, water mills and the city walls. A quarter of the church of St. John was reserved for Muslim use, and abandoned houses and gardens were confiscated and distributed by Abu Ubayda or Khalid among the Muslim troops and their families. Owing to its proximity to the desert steppe, Homs was viewed as a favorable place of settlement for Arab tribesmen and became the first city in Syria to acquire a large Muslim population.
Information about the subsequent conquests in northern Syria is scant and partly contradictory. Khalid was dispatched by Abu Ubayda to conquer Qinnasrin (called Chalcis by the Byzantines) and nearby Aleppo. Khalid routed a Byzantine force led by a certain Minas in the outskirts of Qinnasrin. There, Khalid spared the inhabitants following their appeal and claim that they were Arabs forcibly conscripted by the Byzantines. He followed up by besieging the walled town of Qinnasrin, which capitulated in August/September 638. He and Iyad ibn Ghanm then launched the first Muslim raid into Byzantine Anatolia. Khalid made Qinnasrin his headquarters, settling there with his wife. Khalid was appointed Abu Ubayda's deputy governor in Qinnasrin in 638. The campaigns against Homs and Qinnasrin resulted in the conquest of northwestern Syria and prompted Heraclius to abandon his headquarters at Edessa for Samosata in Anatolia and ultimately to the imperial capital of Constantinople.
Khalid may have participated in the siege of Jerusalem, which capitulated in 637 or 638. According to al-Tabari, he was one of the witnesses of a letter of assurance by Umar to Patriarch Sophronius of Jerusalem guaranteeing the safety of the city's people and property.

Dismissal and death: According to Sayf ibn Umar, later in 638 Khalid was rumored to have lavishly distributed war spoils from his northern Syrian campaigns, including a sum to the Kindite nobleman al-Ash'ath ibn Qays. Umar consequently ordered that Abu Ubayda publicly interrogate and relieve Khalid from his post regardless of the interrogation's outcome, as well as to put Qinnasrin under Abu Ubayda's direct administration. Following his interrogation in Homs, Khalid issued successive farewell speeches to the troops in Qinnasrin and Homs before being summoned by Umar to Medina. Sayf's account notes that Umar sent notice to the Muslim garrisons in Syria and Iraq that Khalid was dismissed not as a result of improprieties but because the troops had become "captivated by illusions on account of him [Khalid]" and he feared they would disproportionately place their trust in him rather than God.
Khalid's sacking did not elicit public backlash, possibly due to existing awareness in the Muslim polity of Umar's enmity toward Khalid, which prepared the public for his dismissal, or because of existing hostility toward the Makhzum in general as a result of their earlier opposition to Muhammad and the early Muslims. In the account of Ibn Asakir, Umar declared at a council of the Muslim army at Jabiya in 638 that Khalid was dismissed for lavishing war spoils on war heroes, tribal nobles and poets instead of reserving the sums for needy Muslims. No attending commanders voiced opposition, except for a Makhzumite who accused Umar of violating the military mandate given to Khalid by Muhammad. According to the Muslim jurist al-Zuhri (d. 742), before his death in 639, Abu Ubayda appointed Khalid and Iyad ibn Ghanm as his successors, but Umar confirmed only Iyad as governor of the Homs–Qinnasrin–Jazira district and appointed Yazid ibn Abi Sufyan governor over the rest of Syria, namely the districts of Damascus, Jordan and Palestine.
Khalid died in Medina or Homs in 21 AH (c. 642 CE). Purported hadiths related about Khalid include Muhammad's urgings to Muslims not to harm Khalid and prophecies that Khalid would be dealt injustices despite his tremendous contributions to Islam. In Islamic literary narratives, Umar expresses remorse over dismissing Khalid and the women of Medina mourn his death en masse. Athamina considers these all to be "no more than latter-day expressions of sympathy on the part of subsequent generations for the heroic character of Khalid as portrayed by Islamic tradition".

Legacy: Khalid is credited by the early sources for being the most effective commander of the conquests, including after his dismissal from the supreme command. He is considered "one of the tactical geniuses of the early Islamic period" by Donner. The historian Carole Hillenbrand calls him "the most famous of all Arab Muslim generals", and Humphreys describes him as "perhaps the most famous and brilliant Arab general of the Riddah wars and the early conquests". In Kennedy's assessment, Khalid was "a brilliant, ruthless military commander, but one with whom the more pious Muslims could never feel entirely comfortable". While recognizing his military achievements, the early Islamic sources present a mixed assessment of Khalid due to his early confrontation with Muhammad at Uhud, his reputation for brutal or disproportionate actions against Arab tribesmen during the Ridda wars and his military fame which disturbed the pious early converts.
According to the historian Richard Blackburn, despite attempts in the early sources to discredit Khalid, his reputation has developed as "Islam's most formidable warrior" during the eras of Muhammad, Abu Bakr and the conquest of Syria. Kennedy notes that "his reputation as a great general has lasted through the generations and streets are named after him all over the Arab world". Khalid is considered a war hero by Sunni Muslims, while many Shia Muslims view him as a war criminal for his execution of Malik ibn Nuwayra and immediate marriage of his widow, in contravention of the traditional Islamic bereavement period.

Family and claimants of descent: Khalid's eldest son was named Sulayman, hence his kunya ('paedonymic') Abu Sulayman ('father of Sulayman'). Khalid was married to Asma, a daughter of Anas ibn Mudrik, a prominent chieftain and poet of the Khath'am tribe. Their son Abd al-Rahman became a reputable commander in the Arab–Byzantine wars and a close aide of Mu'awiya ibn Abi Sufyan, the governor of Syria and later founder and first caliph of the Umayyad Caliphate, serving as the latter's deputy governor of the Homs–Qinnasrin–Jazira district. Another son of Khalid, Muhajir, was a supporter of Ali, who reigned as caliph in 656–661, and died fighting Mu'awiya's army at the Battle of Siffin in 657 during the First Muslim Civil War.