Khalid ibn al-Walid
The annual sum to be paid by al-Hira amounted to 60,000 or 90,000 silver dirhams, which Khalid forwarded to Medina, marking the first tribute the Caliphate received from Iraq.
During the engagements in and around al-Hira, Khalid received key assistance from al-Muthanna ibn Haritha and his Shayban tribe, who had been raiding this frontier for a considerable period before Khalid's arrival, though it is not clear if al-Muthanna's earlier activities were linked to the nascent Muslim state. After Khalid departed, he left al-Muthanna in practical control of al-Hira and its vicinity. He received similar assistance from the Sadus clan of the Dhuhl tribe under Qutba ibn Qatada and the Ijl tribe under al-Madh'ur ibn Adi during the engagements at Ubulla and Walaja. None of these tribes, all of which were branches of the Banu Bakr confederation, joined Khalid when he operated outside of their tribal areas.
Khalid continued northward along the Euphrates valley, attacking Anbar on the east bank of the river, where he secured capitulation terms from its Sasanian commander. 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. Khalid encountered stiff resistance there by the tribesmen of the Namir, compelling him to besiege the town's fortress. The Namir were led by Hilal ibn Aqqa, a Christian chieftain allied with the Sasanians, who Khalid had crucified after defeating him. Ayn al-Tamr capitulated and Khalid captured the town of Sandawda to the north. 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.

Modern assessments: Athamina doubts the Islamic traditional narrative that Abu Bakr directed Khalid to launch a campaign in Iraq, citing Abu Bakr's disinterest in Iraq at a time when the Muslim state's energies were focused principally on the conquest of Syria. Unlike Syria, Iraq had not been the focus of Muhammad's or the early Muslims' ambitions, nor did the Quraysh maintain trading interests in the region dating to the pre-Islamic period as they had in Syria. According to Shaban, it is unclear if Khalid requested or received Abu Bakr's sanction to raid Iraq or ignored objections by the caliph. Athamina notes hints in the traditional sources that Khalid initiated the campaign unilaterally, implying that the return of the Muhajirun in Khalid's ranks to Medina following Musaylima's defeat likely represented their protest of Khalid's ambitions in Iraq. Shaban holds that the tribesmen who remained in Khalid's army were motivated by the prospect of war booty, particularly amid an economic crisis in Arabia which had arisen in the aftermath of the Ridda campaigns.
According to Fred Donner, the subjugation of Arab tribes may have been Khalid's primary goal in Iraq and clashes with Persian troops were the inevitable, if incidental, result of the tribes' alignment with the Sasanian Empire. In Kennedy's view, Khalid's push toward the desert frontier of Iraq was "a natural continuation of his work" subduing the tribes of northeastern Arabia and in line with Medina's policy to bring all nomadic Arab tribes under its authority. 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.
The extent of Khalid's role in the conquest of Iraq is disputed by modern historians. Patricia Crone argues it is unlikely Khalid played any role on the Iraqi front, citing seeming contradictions by contemporary, non-Arabic sources, namely the Armenian chronicle of Sebeos (c. 661) and the Khuzistan Chronicle (c. 680). The former only records Arab armies being sent to conquer Iraq as the Muslim conquest of Syria was already underway—as opposed to before as held by the traditional Islamic sources—while the latter mentions Khalid as the conqueror of Syria only. Crone views the traditional reports as part of a general theme in the largely Iraq-based, Abbasid-era (post-750) sources to diminish the early Muslims' focus on Syria in favor of Iraq. Crone's assessment is considered a "radical critique of the [traditional] sources" by R. Stephen Humphreys, while Khalid Yahya Blankinship calls it "too one-sided ... The fact that Khālid is a major hero in the historical traditions of Iraq certainly suggests ties there that can have come only from his early participation in its conquest".

March to Syria: All early Islamic accounts agree that Khalid was ordered by Abu Bakr to leave Iraq for Syria to support Muslim forces already present there. Most of these accounts hold that the caliph's order was prompted by requests for reinforcements by the Muslim commanders in Syria. Khalid likely began his march to Syria in early April 634. He left small Muslim garrisons in the conquered cities of Iraq under the overall military command of al-Muthanna ibn Haritha.
The chronological sequence of events after Khalid's operations in Ayn al-Tamr is inconsistent and confused. According to Donner, Khalid undertook two further principal operations before embarking on his march to Syria, which have often been conflated by the sources with events that occurred during the march. 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. It is unclear which engagement occurred first, though both were Muslim efforts to bring the mostly nomadic Arab tribes of north Arabia and the Syrian steppe under Medina's control.
In the Dumat al-Jandal campaign, Khalid was instructed by Abu Bakr or requested by one of the commanders of the campaign, al-Walid ibn Uqba, to reinforce the lead commander Iyad ibn Ghanm's faltering siege of the oasis town. Its defenders were backed by their nomadic allies from the Byzantine-confederate tribes, the Ghassanids, Tanukhids, Salihids, Bahra and Banu Kalb. Khalid left Ayn al-Tamr for Dumat al-Jandal where the combined Muslim forces bested the defenders in a pitched battle. 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.
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. 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]". Vaglieri surmises that the oasis was conquered by Iyad ibn Ghanm or possibly Amr ibn al-As as the latter had been previously tasked during the Ridda wars with suppressing Wadi'a, who had barricaded himself in Dumat al-Jandal. Crone, dismissing Khalid's role in Iraq entirely, asserts that Khalid had definitively captured Dumat al-Jandal in the 631 campaign and from there crossed the desert to engage in the Syrian conquest.

Itineraries and the desert march: The starting point of Khalid's general march to Syria was al-Hira, according to most of the traditional accounts, with the exception of al-Baladhuri, who places it at Ayn al-Tamr. 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.