Deployment and appointment of Abu Ubayda to supreme command As the
Ridda wars came to an end, Abu Bakr dispatched three or four armies at different intervals toward
Byzantine Syria to conquer that region. Though there are several versions in the early Islamic tradition, including in the works of
Ibn Hubaysh al-Asadi (),
al-Mas'udi (d. 956),
al-Azdi (d. 944), as well as the 10th-century-compiled
Kitab al-Aghani, that place Abu Ubayda as one of these commanders, modern research, including by historians
H. A. R. Gibb,
C. H. Becker,
Philip K. Hitti,
Andreas Stratos,
D. R. Hill and Khalil Athamina, date Abu Ubayda's dispatch to after Abu Bakr's death. In the comprehensive 9th-century history of the
early Muslim conquests by al-Baladhuri, the latter states "there is no truth" to the claim that Abu Ubayda was sent by Abu Bakr. Rather, the caliph "intended to send Abu ʿUbayda at the head of one of the armies, but the latter asked the caliph to relieve him of this mission". Athamina assesses that "certain allusions" in the Islamic sources offer context to the notion that Abu Ubayda, despite his participation in several expeditions under Muhammad and his high-standing among the Muslims, did not have the desire nor the necessary military experience and merit to accept the post Abu Bakr offered him. Modern research indicates that Abu Ubayda was dispatched to the Syrian front by Abu Bakr's successor, Umar, and early Muslim authors al-Baladhuri,
al-Fasawi (d. 890) and
Ibn Asakir (d. 1175) mention that it was in the capacity as commander of an army of reinforcements. His arrival most likely dated to around 636, shortly after the first Muslim
capture of Damascus in late 635 or during the preparation for the subsequent
Battle of the Yarmuk. At the time, supreme command of the Muslim armies in Syria was held by
Khalid ibn al-Walid with Abu Ubayda playing a supportive role to Khalid. Umar may have sent Abu Ubayda to assume the supreme command. Several accounts in the Islamic tradition claim Abu Ubayda concealed the caliph's order from the rest of the army to avoid potentially insulting Khalid or provoking a mutiny while the Muslims were on the cusp of a major confrontation with the Byzantines. Athamina dismisses the reliability of these claims, considering them militarily illogical and meant to dramatise the change in command and emphasise Abu Ubayda's "moral superiority and unselfishness". Instead, Athamina maintains Abu Ubayda's appointment to the supreme command was made by Umar, who had kept in constant contact with Abu Ubayda through letters and emissaries, after the decisive Muslim victory at the Yarmuk. Abu Ubayda may have been chosen to lead at that time, when the Byzantine defence of Syria had taken an enormous blow, as the circumstances called for an able administrator to take the helm from a military commander like Khalid. The Islamic tradition provides a host of moral and personal reasons why Khalid was demoted in favour of Abu Ubayda, but most modern historians view these as either partially valid or literary innovations. Athamina holds Abu Ubayda was likely installed because Khalid and his large force of tribesmen from Arabia and Iraq, along with their families, presented a threat to the old-established, formerly Byzantine-allied, and militarily experienced Arab tribes of Syria, whose defection was considered vital by Umar to form a network of defences against the Byzantines. This motivated him to demote Khalid and disband his army, the remnants of which were transferred to the
Sasanian front in Iraq.
Ajnadayn and Damascus During 634, the Muslims heard that 90,000 strong
Byzantine army had gathered at Ajnadayn about southwest of
Jerusalem. All the divisions of the Muslim army, about 32,000 in number, joined Khalid at Ajnadayn on 24 July 634. Under the command of Khalid ibn al-Walid the Muslims defeated the
Byzantine army there on 30 July 634 at the
Battle of Ajnadayn. One week later, Khalid, along with Abu Ubayda, moved towards
Damascus. On their way to Damascus, they defeated another Byzantine army at the Battle of Yaqusa in mid-August 634. Caloiis and Azrail, the governor of Damascus, led another army to stop Khalid's corps but they were also defeated in the
Battle of Marj al-Saffar on 19 August 634. The next day the Muslims reached Damascus and
besieged the city. The siege continued for 30 days. After defeating the Byzantine reinforcements sent by
Emperor Heraclius at the
Battle of Sanita-al-Uqab, from Damascus, Khalid's forces attacked and entered the city. With Khalid's divisions
investing the city from the northeast, Thomas, the purported son-in-law of the Emperor Heraclius, surrendered the city to Abu Ubayda, who was besieging the
Bab al-Jabiya (Jabiya Gate), on 19 September 634. Abu Ubayda had been appointed by Khalid ibn al-Walid to lay siege the Jabiya Gate of Damascus. Abu Ubayda, Shurahbil ibn Hasana and 'Amr ibn al-'As put forward a peace proposal to the citizens of Damascus after Khalid ibn al-Walid attacked the city and conquered it by force, as they were unaware of Khalid's attack from the
Eastern Gate. The peace proposal was reluctantly endorsed by Khalid. The Byzantine army was given a cease fire of three days and allowed to go as far as they could with their families and treasure. Others residents simply agreed to stay in Damascus and pay tribute. The Muslims controlled the road to Emesa, so the Byzantines went west and then north up the
Beqaa Valley. After the three-day truce was over, the Muslim cavalry, under Khalid's command, pursued the Byzantine column via the shorter Emesa road and caught up with them in the northwest Beqaa Valley, just before they entered the mountains en route to
Antioch at the
Battle of Marj al-Dibaj.
Conquest of central Syria Soon after the appointment of Abu Ubayda as commander-in-chief, he sent a small detachment to the annual fair held near
Abla, close to
Zahlé east of
Beirut. A Byzantine and
Christian Arab force present in the area confronted the detachment, whose size had been miscalculated by Muslim informants, and quickly encircled it. Before the detachment could be destroyed, Abu Ubayda, having received new intelligence, sent Khalid to reinforce them. Khalid reached the area in time, broke the encirclement, and defeated the opposing forces on 15 October 634, returning with significant booty from the fair and a number of prisoners. With central Syria captured, the Muslims had given the Byzantines a decisive blow. For the Byzantines, communications between northern Syria and
Palestine were now cut off. Abu Ubayda decided to march to Fahl (
Pella), which is about below
sea level, and where a strong Byzantine garrison and survivors of the Battle of Ajnadayn were present. The region was crucial because from there the Byzantine army could strike eastwards and cut communications lines with
Arabia. Moreover, with this large garrison at the rear, Palestine could not be invaded. Thus the Muslim army moved to Fahl. The Byzantine army was defeated at the
Battle of Fahl on 23 January 635.
Battles for Emesa and the second battle of Damascus After the battle, which would prove to be a key to Palestine and
Jordan, the Muslim armies split up. Shurahbeel and Amr's corps moved south to capture Palestine. Meanwhile, Abu Ubayda and Khalid, with a relatively larger corps, moved north to conquer
Lebanon and northern Syria. While the Muslims were occupied at Fahl, Heraclius, sensing the opportunity, quickly sent an army under General
Theodore Trithyrius to recapture Damascus, where a small Muslim garrison had been left. Shortly after Heraclius dispatched this new army, the Muslims, having finished the business at Fahl, were on their way to Emesa. The Byzantine army met the Muslims halfway to Emesa, at Maraj al-Rome. During the night, Theodras sent half of his army towards Damascus to launch a surprise attack on the Muslim garrison. Khalid's spy informed him about the move. Khalid, having received permission from Abu Ubayda, galloped towards Damascus with his
mobile guard. While Abu Ubayda fought and defeated the Byzantine army in the
Battle of Marj ar-Rum, Khalid moved to Damascus with his
light cavalry and attacked and defeated Theodras in the
second battle of Damascus. A week later, Abu Ubayda moved towards
Heliopolis (Baalbek), where the great
Temple of Jupiter stood. Heliopolis surrendered to Muslim rule after little resistance and agreed to pay tribute. Abu Ubayda sent Khalid towards Emesa. Emesa and
Chalcis sued for peace for a year. Abu Ubayda accepted the offer. So rather than invading the districts of Emesa and Chalcis, he consolidated his rule in conquered land and captured
Hama,
Maarrat al-Nu'man. The peace treaties were, however, on Heraclius's instructions, to slow down the Muslim advance and to secure time for the preparation of the defences across northern Syria (present-day Lebanon, Syria and southern
Turkey). Having mustered sizeable armies at Antioch, Heraclius sent them to reinforce strategically important areas of northern Syria, such as
Emesa and Chalcis. With the arrival of Byzantine army in the city, the peace treaty was violated. So Abu Ubadiah and Khalid marched to Emesa, and the Byzantine army that had halted Khalid's advance guard was defeated. The Muslims
besieged Emesa, which was finally conquered in March 636 after a six months' siege.
Battle of Yarmouk After capturing Emesa, the Muslims moved north to capture the whole of the northern Syria. Khalid, acting as an advance guard raided northern Syria. At Shaizer, Khalid intercepted a convoy taking provisions for Chalcis. The prisoners were interrogated and informed him about Emperor Heraclius' ambitious plan to take back Syria. They told him that an army, possibly 200,000 strong, would soon emerge to recapture their territory. After his past experience fighting the Muslim armies, Heraclius, was avoiding pitch battles with the Muslims. He planned to send massive reinforcements to all the major cities and isolate the Muslim corps from each other, and thus separately encircle and destroy the Muslim armies. In June 636, five massive armies were sent by Heraclius to recapture Syria. Khalid, sensing Heraclius's plan, feared that the Muslim armies would be isolated and destroyed. In a
council of war he suggested that Abu Ubayda draw all the Muslim armies to one place so as to fight a decisive battle with the Byzantines. Agreeing with Khalid's suggestion, Abu Ubayda ordered all the Muslim armies in Syria to evacuate the lands they had conquered and meet at
Jabiya. This manoeuvre turned out to be a decisive blow to the Heraclius's plan, as he did not wish engage his troops in an open battle with the Muslims, where their light cavalry could be effectively used. On Khalid's suggestion, Abu Ubayda ordered the Muslim army to withdraw from Jabiya to the plain on the
Yarmouk River, where cavalry could be used. While the Muslim armies were gathering at Yarmouk, Khalid intercepted and routed the Byzantine advance guard. This was to ensure the safe retreat of the Muslims to Yarmouk. The Muslim armies reached Yarmouk in July 636. Around mid July, the Byzantine army arrived. The Byzantine commander in chief, Vahan, sent Christian Arab troops of the Ghassanid king,
Jabalah ibn al-Aiham, to check the strength of the Muslims. Khalid's mobile guard defeated and routed the Christian Arabs; this was the last action before the battle started. For the next month negotiations continued between the two armies, and Khalid went to meet Vahan in person at Byzantine camp. Meanwhile, the Muslims received reinforcements sent by Caliph Umar. Finally on 15 August, the Battle of Yarmouk was fought. It lasted for 6 days and ended in a devastating defeat for the Byzantines. The Battle of Yarmouk is considered to be a historic defeat that sealed the fate of Byzantines in the Middle East. The magnitude of defeat was so intense that Byzantine would never recover from it. It potentially left the whole of the Byzantine Empire vulnerable to Muslim attack.
Siege of Jerusalem With the Byzantine army shattered and routed, the Muslims quickly recaptured the territory that they conquered prior to Yarmouk. Abu Ubayda held a meeting with his high command, including Khalid, to decide on future conquests. They decided to conquer Jerusalem. The Siege of Jerusalem lasted four months after which the city agreed to surrender, but only to caliph Umar in person. 'Amr ibn al-'As suggested that Khalid should be sent as if he was the caliph, because of his very strong resemblance to Umar. However, Khalid was recognized. So the surrender of Jerusalem was delayed until Umar arrived with
Jerusalem formally surrendering on April 637. After capture of Jerusalem, the Muslim armies separated once again. Yazid's corps went to Damascus and captured Beirut. Amr and Shurahbil's corps went on to conquer the rest of Palestine, while Abu Ubayda and Khalid, at the head of a 17,000 strong army moved north to conquer whole of the northern Syria. Abu Ubayda sent the commanders 'Amr ibn al-'As,
Yazid ibn Abi Sufyan, and Shurahbil ibn Hassana back to their areas to reconquer them. Most of the areas submitted without a fight. Abu Ubayda, along with Khalid, moved to northern Syria to reconquer that area with a 17,000 strong army. Khalid along with his cavalry was sent to Hazir and Abu Ubayda moved to Chalcis.
Conquest of northern Syria Defence of Emesa After the devastating defeat in the Battle of Yarmouk, the remainder of the Byzantine empire was left vulnerable. With few military resources left, it was no longer in a position to attempt a military comeback in Syria. To gain time to prepare a defence of the rest of his empire, Heraclius needed to keep the Muslims occupied in Syria. Heraclius thus sought help from the Christian Arab tribes based in Jazirah and were mainly from Circesium and Hīt. The tribes mustered a large army and marched to
besiege Emesa. Abu Ubayda withdrew all his forces from northern Syria to Emesa as a part of complex strategy which he devised to repel the invasion of the Christian Arab forces against Emesa. Caliph
Umar instructed Abu Ubaydah to send field commanders outside Emesa with sufficient forces to lay counter siege to cities in
Jazira, homeland of enemy Arab Christian tribes, in order to divert the focus of enemy concentration in Emesa. So in 638 the Muslim forces under
Iyad ibn Ghanm attacked Hīt, which they found to be well fortified. So they left some of the army to lay siege to the city, while the rest of the army went on to Circesium. Khalid was in favour of an open battle outside the fort, but Abu Ubayda sought Umar's advice, who handled the situation brilliantly. Umar sent detachments of the Muslim army from Iraq to invade Al-Jazira, homeland of the invading Christian Arabs, from three different routes. Moreover, another detachment was sent to Emesa from Iraq under Al-Qa'qa ibn Amr, a veteran of Yarmouk who had been sent to Iraq for the
Battle of al-Qadisiyyah. Umar himself marched from Medina at the head of 1,000 men. The Christian Arabs, when they received the news of the Muslim invasion of their homeland, abandoned the siege and hastily withdrew to Al-Jazira. At this point Khalid and his mobile guard came out of the fort and devastated the army, attacking them from the rear.
After Emesa Abu Ubayda sent Khalid, with his elite cavalry, the mobile guard, towards Chalcis. The fort was guarded by the Greek troops under their commander,
Menas, who was reported to be of high prestige, second only to the emperor himself. Menas, diverting from conventional Byzantine tactics, decided to face Khalid and destroy the leading elements of the Muslim army before the main body could join them at Hazir, east of Chalcis. This resulted in the
Battle of Hazir, which led to Umar praising Khalid's military genius. Umar is reported to have said: .
Rawdat al-safa (Garden of felicity) by Mirkhwand (d. 1498), Shiraz, 1571-72 (Arthur M. Sackler Collection, S1986.241). Abu Ubayda soon joined Khalid at the virtually impregnable fort of Chalcis, which surrendered in June 637. With this strategic victory, the territory north of Chalcis lay open to the Muslims. Khalid and Abu Ubayda continued their march northward and laid
siege to Aleppo, which was captured after fierce resistance from desperate Byzantine troops in October 637. The next objective was the splendid city of Antioch, the capital of the Asian zone of the Byzantine Empire. Before marching towards Antioch, Khalid and Abu Ubayda decided to isolate the city. Accordingly, they sent detachments north to eliminate all possible Byzantine forces and captured the garrison town of
Azaz, from
Aleppo. From there the Muslims attacked Antioch on the eastern side. A desperate battle was fought between the Muslim army and the defenders of Antioch, popularly known as
Battle of Iron Bridge. The Byzantine army was composed of the survivors of Yarmouk and other Syrian campaigns. After being defeated, the Byzantines retreated to Antioch and the Muslims besieged the city. Having little hope of help from Emperor Heraclius, Antioch surrendered on 30 October 637, with the terms that all Byzantine troops would be given safe passage to
Constantinople. Abu Ubayda sent Khalid northwards, while he marched south and captured Lazkia, Jabla,
Tartus and the coastal areas west of
Anti-Lebanon Mountains. Khalid moved north and raided territory up to the
Kızıl River (Kızılırmak) in Anatolia. Emperor Heraclius had already left Antioch for
Edessa before the arrival of the Muslims. He arranged for the necessary defences in
Al-Jazira and Armenia and left for his capital, Constantinople. On his way to Constantinople he had a narrow escape when Khalid, after the capturing Marash, was heading south towards
Munbij. Heraclius hastily took the mountainous path and, passing through the
Cilician Gates, is reported to have said:
Campaigns in Armenia and Anatolia After the battle Umar ordered the conquest of Al-Jazira, which was completed by late summer 638. Following the victory, Abu Ubayda sent Khalid and
Iyad ibn Ghanm (conqueror of Al-Jazira) to invade the Byzantine territory north of Al-Jazira. They marched independently and captured Edessa,
Amida (Diyarbakır),
Malatya and whole of Armenia up to
Ararat and raided northern and central Anatolia. Heraclius had already abandoned all the forts between Antioch and Tartus to create a
buffer zone or
no man's land between Muslim controlled areas and mainland Anatolia. Umar for the time being stopped his armies from invading invasion deeper into Anatolia. Rather, he ordered Abu Ubayda, now the governor of Syria, to consolidate his rule in Syria. At this point, Umar is reported to have said: The expedition to Anatolia and Armenia marked the end of the military career of Khalid. Due to the dismissal of Khalid from the army and a famine and plague the next year, the Muslim armies were kept from invading Anatolia. ==Relief efforts during the 638 famine==