The Expedition of Khalid ibn Sa'id was a military campaign conducted by the Rashidun Caliphate against the Byzantine Empire in late 633 or in early 634, under the command of Khalid ibn Sa’id. The Muslims were able to obtain initial success by defeating a Byzantine-allied Arab christian force at Tayma. However, the campaign ultimately ended in a disastrous failure, as Khalid was eventually defeated by the Byzantine commander Vahan at Marj al-Saffar, south of Damascus in Syria.
Background
Following the end of the Ridda Wars, Abu Bakr consolidated and expanded his realm within the entire Arabian Peninsula. He sent the companion of Muhammad, Khalid ibn Sa'id with a preliminary volunteer force to set up camp at Tayma, strictly ordering him not to initiate battle or march into enemy territory unless explicitly authorized, and to act as a buffer while gathering local Bedouin tribes and closely observe any Byzantine movements. Khalid initially advanced with his own retinue, after which a string of reinforcements gradually joined him. One of these additional contingents was led by Al-Walid ibn Uqba. Khalid was consequently able to concentrate a sizeable army in the region, plausibly over 15,000 strong.
Campaign
Opening moves
While stationed at Tayma, Khalid observed the local Christian-Arab tribes mobilizing under the Byzantine allied chieftain Mahan. The Byzantines had learned of the Rashidun buildup near Tayma, and consequently began mustering their own forces. The Byzantine army consisted of both regular troops and significant quantities of Arab foederati, who were drawn from the Ghassanid, Tanukhid, Salib, Judham and even Lakhmid tribal groupings, likely exceeding 20,000. On the basis of al-Tabari, Emperor Heraclius resided at Jerusalem at the time of Khalid's expedition, having made a pilgrimage there following the end of the war with Persia. It is likely that he was the mastermind of Roman operations at the time, with part of the reason behind his journey to Palestine being to strengthen the frontier defences and amass a powerful Byzantine army in the area. When Khalid learned of the Byzantine preparations, he dispatched a letter to Abu Bakr, who ordered Khalid to conduct a pre-emptive attack against the Byzantines. The first forces to confront Khalid were the Arab foederati in Byzantine service. However, upon witnessing Khalid's impressive army drawn up in combat formation, many of these foederati defected to the Rashiduns and embraced Islam. The remainder of the Byzantine army retreated following the betrayal.
Battle of Al Qastal
Khalid dispatched a messenger to inform the caliph of this development, and Abu Bakr responded with an order for Khalid to advance but to avoid attacking too deep into Byzantine territory, so as not to allow his army to be cut off. Khalid promptly marched towards the border with the empire and arrived near the site of Al Qastal. There he met the Byzantine army, which had regrouped under the command of the Armenian general Vahan. This time a battle was joined, wherein the Arabs gained the upper hand and defeated the Byzantine army. However, the Rashiduns sustained severe casualties even in victory, prompting Khalid to dispatch a request to Abu Bakr for reinforcements. Khalid's warriors also sought to retire from service under him, possibly dissatisfied with his leadership or exhausted after the battle of Al Qastal. However, the caliph duly granted Khalid's request with the dispatch of elite warriors from Yemen. This replacement army included two prominent commanders, Dhu al-Kala and Ikrima ibn Amr.
Battle of Marj al-Saffar
Once the supplementary Yemeni troops arrived, Khalid ibn Sa'id immediately sought to exploit his earlier victory and begin a rapid advance towards Damascus. When the Rashidun army approached the opposing Byzantine forces, who had once again regrouped, Vahan feigned flight in the direction of Damascus to lure Khalid ibn Sa'id to press further in his bold advance into Syria. At Marj al-Saffar Vahan's forces suddenly turned about to fight the Rashiduns, while hidden troops posted previously in ambushing positions emerged and began surrounding the Muslim flanks. The Byzantines had concealed two unknown droungarioi commanding 4,000 men apiece, which along with the contingent feigning flight under Vahan himself, may have formed a total strength of around 16,000 men for the Byzantine army at Marj al-Saffar. During the unexpected counterattack, Khalid's son, Sa'id ibn Khalid, was killed. Khalid himself fled the battlefield with the remnants of his troops routing with him. However, Ikrima was able to rally 6,000 warriors to perform an effective fighting retreat, preventing the total annihilation of the Rashidun army. Thus, the Byzantines had managed to repulse the first major Rashidun offensive into the Levant, though they had not been able to fully exploit their victory.
Aftermath
Following the disaster at Marj al-Saffar, Khalid retreated with 3,000 Rashidun survivors as far south as Wadi al-Qura. Ikrima and his 6,000 strong contingent, having narrowly avoided destruction via their rear-guard action, withdrew southwards, likely to the area between Ma'an and Ma'ab. Around this time, Shurahbil ibn Hasana had been dispatched back to the caliph by Khalid ibn al-Walid, who was engaged in fighting against the Sasanian Empire along the Mesopotamian frontier. Abu Bakr permitted Shurahbil to travel to Wadi al-Qura to collect some of the surviving troops under Khalid ibn Sa'id to add to his own retinue. Shurahbil journeyed to al-Qura, where he took most of the troops from Khalid ibn Sa'id. Abu Bakr mustered a new army and appointed Yazid ibn Abi Sufyan and his brother Mu'awiya as commanders, with the rest of Khalid ibn Sa'id's troops being assigned to him. Meanwhile, Ikrima and his troops would go on serve as part of next large Rashidun offensive against the Byzantines, attached to Shurahbil's column, in late 633 or early 634.