Abu Bakr later reassigned him to command the Muslim armies in Syria, where he led his forces on an unconventional march across a long, waterless stretch of the Syrian Desert, boosting his reputation as a military strategist.

[15] The historian Akram Diya Umari holds that Khalid and Amr embraced Islam and relocated to Medina following the Treaty of Hudaybiyya, apparently after the Quraysh dropped demands for the extradition of newer Muslim converts to Mecca.

In the view of Leone Caetani and Bernard Lewis, the opposing tribes who had established ties with Medina regarded their religious and fiscal obligations as being a personal contract with Muhammad; their attempts to negotiate different terms after his death were rejected by Abu Bakr, who proceeded to launch the campaigns against them.

[45] According to the account of the 8th-century historian Sayf ibn Umar, Malik had also been cooperating with the prophetess Sajah, his kinswoman from the Yarbu, but after they were defeated by rival clans from the Tamim, left her cause and retreated to his camp at al-Butah.

[47] The modern historian Wilferd Madelung discounts Sayf's version, asserting that Umar and other Muslims would not have protested Khalid's execution of Malik if the latter had left Islam,[48] while Watt considers accounts about the Tamim during the Ridda in general to be "obscure ... partly because the enemies of Khālid b. al-Walīd have twisted the stories to blacken him".

[47] Following a series of setbacks in her conflict with rival Tamim factions, Sajah joined the strongest opponent of the Muslims: Musaylima, the leader of the sedentary Banu Hanifa tribe in the Yamama,[35][37] the agricultural eastern borderlands of Najd.

[51] After Muhammad died, support for Musaylima surged in the Yamama,[52] whose strategic value lay not only with its abundance of wheat fields and date palms, but also its location connecting Medina to the regions of Bahrayn and Oman in eastern Arabia.

[55] Ikrima was repelled by Musaylima's forces and thereafter instructed by Abu Bakr to quell rebellions in Oman and Mahra (central southern Arabia) while Shurahbil was to remain in the Yamama in expectation of Khalid's large army.

[35] Khalid assigned a Hanifite taken captive early in the campaign, Mujja'a ibn al-Murara, to assess the strength, morale and intentions of the Hanifa in their Yamama fortresses in the aftermath of Musaylima's slaying.

[7] According to Lecker, Khalid and the other Qurayshite generals "gained precious experience [during the Ridda wars] in mobilizing large multi-tribal armies over long distances" and "benefited from the close acquaintance of the Kuraysh [sic] with tribal politics throughout Arabia".

[65] According to the historian Khalil Athamina, the remnants of Khalid's army consisted of nomadic Arabs from Medina's environs whose chiefs were appointed to replace the vacant command posts left by the sahaba ('companions' of Muhammad).

[65] The historian Fred Donner holds that the Muhajirun and the Ansar still formed the core of his army, along with a large proportion of nomadic Arabs likely from the Muzayna, Tayy, Tamim, Asad and Ghatafan tribes.

[72][73] Al-Hira's Arab tribal nobles, many of whom were Nestorian Christians with blood ties to the nomadic tribes on the city's western desert fringes, barricaded in their scattered fortified palaces.

[72] Afterward, he plundered the surrounding market villages frequented by tribesmen from the Bakr and Quda'a confederations, before moving against Ayn al-Tamr, an oasis town west of the Euphrates and about 90 kilometers (56 mi) south of Anbar.

[72] By this stage, Khalid had subjugated the western areas of the lower Euphrates and the nomadic tribes, including the Namir, Taghlib, Iyad, Taymallat and most of the Ijl, as well as the settled Arab tribesmen, which resided there.

[68] Madelung asserts Abu Bakr relied on the Qurayshite aristocracy during the Ridda wars and early Muslim conquests and speculates that the caliph dispatched Khalid to Iraq to allot the Makhzum an interest in that region.

One of the operations was against Dumat al-Jandal and the other against the Namir and Taghlib tribes present along the western banks of the upper Euphrates valley as far as the Balikh tributary and the Jabal al-Bishri mountains northeast of Palmyra.

[94] Afterward, Khalid executed the town's Kindite leader Ukaydir, who had defected from Medina following Muhammad's death, while the Kalbite chief Wadi'a was spared after the intercession of his Tamimite allies in the Muslims' camp.

[95] The historians Michael Jan de Goeje and Caetani dismiss altogether that Khalid led an expedition to Dumat al-Jandal following his Iraqi campaign and that the city mentioned in the traditional sources was likely the town by the same name near al-Hira.

[27] The historian Laura Veccia Vaglieri calls their assessment "logical" and writes that "it seems impossible that Khālid could have made such a detour which would have taken him so far out of his way while delaying the accomplishment of his mission [to join the Muslim armies in Syria]".

[103] In the Dumat al-Jandal–Damascus route, such placenames exist, namely the sites of Qulban Qurajir, associated with 'Quraqir', along the eastern edge of Wadi Sirhan, and Sab Biyar, which is identified with Suwa 150 kilometers (93 mi) east of Damascus.

[105] Lynch holds that the story of the march, which "would have excited and entertained" Muslim audiences, was created out of "fragments of social memory" by inhabitants who attributed the conquests of their towns or areas to Khalid as a means "to earn a certain degree of prestige through association" with the "famous general".

[109] By the time Khalid had left Iraq, the Muslim armies in Syria had already fought a number of skirmishes with local Byzantine garrisons and dominated the southern Syrian countryside, but did not control any urban centers.

[111] A single account in al-Baladhuri instead attributes Khalid's appointment to a consensus among the commanders already in Syria, though Athamina asserts "it is inconceivable that a man like [Amr ibn al-As] would agree" to such a decision voluntarily.

[127] 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.

[128] Although several versions of Khalid's treaty were recorded in the early Muslim and Christian sources,[c] they generally concur that the inhabitants' lives, properties and churches were to be safeguarded, in return for their payment of the jizya (poll tax).

[140] 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.

[151] 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.

[148] 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.

[17] 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.