Khalid ibn al-Walid
The segment of the general march called the 'desert march' by the sources occurred at an unclear stage after the al-Hira departure. This phase entailed Khalid and his men—numbering between 500 and 800 strong—marching from a well called Quraqir across a vast stretch of waterless desert for six days and five nights until reaching a source of water at a place called Suwa. As his men did not possess sufficient waterskins to traverse this distance with their horses and camels, Khalid had some twenty of his camels increase their typical water intake and sealed their mouths to prevent the camels from eating and consequently spoiling the water in their stomachs; each day of the march, he had a number of the camels slaughtered so his men could drink the water stored in the camels' stomachs. The utilization of the camels as water storage and the locating of the water source at Suwa were the result of advice given to Khalid by his guide, Rafi ibn Amr of the Tayy.
Excluding the above-mentioned operations in Dumat al-Jandal and the upper Euphrates valley, the traditional accounts agree on only two events of Khalid's route to Syria after the departure from al-Hira: the desert march between Quraqir and Suwa, and a subsequent raid against the Bahra tribe at or near Suwa and operations which resulted in the submission of Palmyra; otherwise, they diverge in tracing Khalid's itinerary. Based on these accounts, Donner summarizes three possible routes taken by Khalid to the vicinity of Damascus: two via Palmyra from the north and the one via Dumat al-Jandal from the south. Kennedy notes the sources are "equally certain" in their advocacy of their respective itineraries and there is "simply no knowing which version is correct".
In the first Palmyra–Damascus itinerary, Khalid marches upwards along the Euphrates—passing through places he had previously reduced—to Jabal al-Bishri and from there successively moves southwestwards through Palmyra, al-Qaryatayn and Huwwarin before reaching the Damascus area. In this route the only span where a desert march could have occurred is between Jabal al-Bishri and Palmyra, though the area between the two places is considerably less than a six-day march and contains a number of water sources. The second Palmyra–Damascus itinerary is a relatively direct route between al-Hira and Palmyra via Ayn al-Tamr. The stretch of desert between Ayn al-Tamr and Palmyra is long enough to corroborate a six-day march and contains scarce watering points, though there are no placenames that can be interpreted as Quraqir or Suwa. 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. The span between the two sites is arid and corresponds with the six-day march narrative.
The desert march is the most celebrated episode of Khalid's expedition and medieval Futuh ('Islamic conquests') literature in general. Kennedy writes that the desert march "has been enshrined in history and legend. Arab sources marvelled at his [Khalid's] endurance; modern scholars have seen him as a master of strategy." He asserts it is "certain" Khalid embarked on the march, "a memorable feat of military endurance", and  "his arrival in Syria was an important ingredient of the success of Muslim arms there". The historian Moshe Gil calls the march "a feat which has no parallel" and a testament to "Khalid's qualities as an outstanding commander".
The historian Ryan J. Lynch deems Khalid's desert march to be a literary construct by the authors of the Islamic tradition to form a narrative linking the Muslim conquests of Iraq and Syria and presenting the conquests as "a well-calculated, singular affair" in line with the authors' alleged polemical motives. 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".

Conquest of Syria: Most traditional accounts have the first Muslim armies deploy to Syria from Medina at the beginning of 13 AH (early spring 634). The commanders of the Muslim armies were Amr ibn al-As, Yazid ibn Abi Sufyan, Shurahbil ibn Hasana and Abu Ubayda ibn al-Jarrah, though the last may have not deployed to Syria until after Umar's succession to the caliphate in the summer of 634, following Abu Bakr's death. According to Donner, the traditional sources' dating of the first Muslim armies' deployment to Syria was behind by several months. It most likely occurred in the autumn of 633, which better conforms with the anonymous Syriac Chronicle of 724, which dates the first clash between the Muslim armies and the Byzantines to February 634. 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.
Khalid was appointed supreme commander of the Muslim armies in Syria. Accounts cited by al-Baladhuri, al-Tabari, Ibn A'tham, al-Fasawi (d. 987) and Ibn Hubaysh al-Asadi hold that Abu Bakr appointed Khalid supreme commander as part of his reassignment from Iraq to Syria, citing the general's military talents and record. 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. Upon his accession, Umar may have confirmed Khalid as supreme commander.
Khalid reached the meadow of Marj Rahit north of Damascus after his army's trek across the desert. He arrived on Easter day of that year, i.e. 24 April 634, a rare precise date cited by most traditional sources, which Donner deems to be likely correct. There, Khalid attacked a group of Ghassanids celebrating Easter before he or his subordinate commanders raided the Ghouta agricultural belt around Damascus. Afterward, Khalid and the commanders of the earlier Muslim armies, except for Amr, assembled at Bosra southeast of Damascus. The trading center of Bosra, along with the Hauran region in which it lies, had historically supplied the nomadic tribes of Arabia with wheat, oil and wine and had been visited by Muhammad during his youth. The Byzantines may not have reestablished an imperial garrison in the city in the aftermath of the Sasanian withdrawal in 628 and the Muslim armies encountered token resistance during their siege. Bosra capitulated in late May 634, making it the first major city in Syria to fall to the Muslims.
Khalid and the Muslim commanders headed west to Palestine to join Amr as the latter's subordinates in the Battle of Ajnadayn, the first major confrontation with the Byzantines, in July. The battle ended in a decisive victory for the Muslims and the Byzantines retreated toward Pella ('Fahl' in Arabic), a major city east of the Jordan River. The Muslims pursued them and scored another major victory at the Battle of Fahl, though it is unclear if Amr or Khalid held overall command in the engagement.

Siege of Damascus: The remnants of the Byzantine forces from Ajnadayn and Fahl retreated north to Damascus, where the Byzantine commanders called for imperial reinforcements. Khalid advanced, possibly besting a Byzantine unit at the Marj al-Suffar plain before besieging the city. Each of the five Muslim commanders were charged with blocking one of the city gates; Khalid was stationed at Bab Sharqi (the East Gate). A sixth contingent positioned at Barzeh immediately north of Damascus repulsed relief troops dispatched by the Byzantine emperor Heraclius (r. 575–641).
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.