Khalid ibn al-Walid
Campaigns in Iraq

With the Yamama pacified, Khalid marched northward toward Sasanian territory in Iraq (lower Mesopotamia). He reorganized his army, possibly because the bulk of the Muhajirun may have withdrawn to Medina. 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). 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. The commanders of the tribal contingents appointed by Khalid were Adi ibn Hatim of the Tayy and Asim ibn Amr of the Tamim. He arrived at the southern Iraqi frontier with about 1,000 warriors in the late spring or early summer of 633.

The focus of Khalid's offensive was the western banks of the Euphrates river and the nomadic Arabs who dwelt there. The details of the campaign's itinerary are inconsistent in the early Muslim sources, though Donner asserts that "the general course of Khalid's progress in the first part of his campaigning in Iraq can be quite clearly traced". The 9th-century histories of al-Baladhuri and Khalifa ibn Khayyat hold Khalid's first major battle in Iraq was his victory over the Sasanian garrison at Ubulla (the ancient Apologos, near modern Basra) and the nearby village of Khurayba, though al-Tabari (d. 923) considers attribution of the victory to Khalid as erroneous and that Ubulla was conquered later by Utba ibn Ghazwan al-Mazini. Donner accepts the town's conquest by Utba "somewhat later than 634" is the more likely scenario, though the historian Khalid Yahya Blankinship argues "Khālid at least may have led a raid there although actually reduced the area".

From Ubulla's vicinity, Khalid marched up the western bank of the Euphrates where he clashed with the small Sasanian garrisons who guarded the Iraqi frontier from nomadic incursions. The clashes occurred at Dhat al-Salasil, Nahr al-Mar'a (a canal connecting the Euphrates with the Tigris immediately north of Ubulla), Madhar (a town several days north of Ubulla), Ullays (likely the ancient trade center of Vologesias) and Walaja. The last two places were in the vicinity of al-Hira, a predominantly Arab market town and the Sasanian administrative center for the middle Euphrates valley.

Al-Hira's capture was the most significant gain of Khalid's campaign. After besting the city's Persian cavalry under the commander Azadhbih in minor clashes, Khalid and part of his army entered the unwalled city. 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. In the meantime, the other part of Khalid's army harried the villages in al-Hira's orbit, many of which were captured or capitulated on tributary terms with the Muslims. The Arab nobility of al-Hira surrendered in an agreement with Khalid whereby the city paid a tribute in return for assurances that al-Hira's churches and palaces would not be disturbed. 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.
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