The Muslim conquest of the Levant occurred from 634 to 638 AD when the Arab Rashidun Caliphate invaded the Levant from the Arabian Peninsula and conquered the region from the Byzantine Empire. The conquest of the Levant by the Arabs was an epoch-making moment, as it led to the formation of a tri-religious (Muslim, Christian, and Jewish) society in the Levant which would have a lasting impact on the history of the region, most notably in the Crusades, the Arab-Israeli Conflict, and the War on Terror.
Background[]
In 628, the Islamic prophet Muhammad sent Arab envoys to the ummah's neighboring states, urging their leaders to convert to Islam. Shah Khusrau II of Persia insulted the Muslim envoy, while the one sent to the Byzantine emperor Heraclius was killed in Ghassanid territory. Muhammad retaliated by sending an army to invade Ghassanid territory, only for this army to be defeated at the Battle of Mu'tah in September 629. Muhammad decided to lead a 30,000-strong punitive expedition, taking the city of Tabuk from the Byzantines. By 631, most of the Arabian Peninsula was under Muslim control. Muhammad died a year later, and Abu Bakr succeeded him as leader of the Muslims with the title of "Caliph", becoming the first ruler of the Rashidun Caliphate.
War[]
First invasion[]

During 633, the Muslims sent four separate corps to invade Palestine, in addition to the areas around the Sea of Galilee, the River Jordan, and the Dead Sea. Though they achieved success, assaults on the large urban centers of the region could not be considered until reinforcements were brought up. In 634, Caliph Abu Bakr sent the order for Khalid ibn al-Walid to move west with his battle-hardened army, marching through the Syrian desert. He had 20 camels drink large amounts of water so that they could be used as makeshift storage tanks; he had them periodically slaughtered so that his men could drink. His 9,000-strong Muslim army defeated a Ghassanid army at Marj Rahit on 24 April 634 as they celebated Easter, and he then turned south towards Bosra, where the arrival of his reinforcements led to its capture by July of 634. Heraclius, who was now in Emesa (Homs), send his brother Theodore and the Armenian general Vardan south to Ajnadayn (25 miles southeast of Jerusalem), where they began to gather a large army. This army was crushed at the Battle of Ajnadayn, after which Theodore was sent back to Constantinople in disgrace. The remnants of the Byzantine army and the local Roman populations withdrew to the walled cities, and Heraclius then withdrew further north from Emesa to Antioch to evade the Muslim forces in the countryside.
Siege of Damascus[]

A week later, the Arabs began to march on Damascus, leaving a mounted detachment at Pella. They defeated a Byzantine blocking force at the Battle of Yaqusa and another army at the Battle of Marj al-Saffar, clearing the road to Damascus. Damascus was then besieged, and Khalid defeated a relief army and several sortie attempts led by Heraclius' son-in-law Thomas. Thomas ultimately surrendered the city to the Arabs, and the city's fall delivered Syria to the Arabs, cut off Byzantine land access to the province of Aegyptus, and rendered Egypt and Palestine vulnerable to Arab attacks. Heraclius started sending orders to the provinces in order to bring more reinforcements to the region. Meanwhile, Umar became Caliph of the Rashiduns, and he created new administrative positions in the provinces and created a more centralized army. He relieved Khalid of his post and appointed Abu Ubaydah ibn al-Jarrah in his place due to previous animosity between the two men; however, guided by the Greek traitor Jonah the Lover, Khalid pursued Thomas to the south of Latakia and fought him at the Battle of Maraj-al-Debaj. Khalid killed Thomas and destroyed the remnants of the Byzantine Damascus garrison, and he returned to Damascus in early October. There, Khalid accepted his demotion without much protest, but his successor Abu Ubaidah ibn al-Jarrah was slower than Khalid due to Umar's close control over operations and his stationing of informants in the Arab army. The Muslims then received some reinforcements, bringing their strength up to 30,000.
Byzantine counterattack[]

The military situation in 635
As the Arabs were getting used to their new command structure, Heraclius brought in reinforcements by land to Antioch and by sea to the Levantine ports of Jaffa, Caesarea, and Acre. This group was commanded by Theodore Trithyrius, the treasurer of the empire, and it assembled to the west of Pella and planned to cut off communication links with Arabia in December 634. In January 635, Abu Ubaidah marched south towards Pella, leading a corps under Yazid ibn Abi Sufyan behind. As soon as the garrison of Pella learned of this, it retreated toward the main army, and battle was joined at the Battle of Fahl. The Muslims defeated a Byzantine night attack and inflicted heavy losses on their Roman foes, forcing them to retreat to Baisan. The Roman army then dispersed into various garrisons to the west and south, while Theodore returned to Antioch by sea. Abu Ubaidah then divided his army into corps to conquer as many cities as possible, with Shurahbil taking Baisan and Tiberias, and Shurahbil and Amr ibn al-As then marched south. By March 635, the Muslims were in control of the whole region south of Beirut, save for Caesarea (which withstood a siege reinforced by Heraclius) and Jerusalem (which had the strongest fortifications). Heraclius then recruited men in order to counterattack in 636, and he then allied with Shah Yazdegerd III, who married Heraclius' granddaughter. The Persians planned to attack the Arabs from the east as Heraclius counterattacked in the north. Meanwhile, Abu Ubaydah's 15,000 troops moved north, and, by November, took over the territory between Damascus and Emesa (Homs), putting Emesa in danger. Heraclius rushed reinforcements to the garrison, which was then besieged. The city fell in March of 636 AD after the Byzantines launched two failed sorties which left their commander and most of the defenders dead.

The Byzantine counterattack in 636
To the north, Heraclius had been preparing a counterattack for some time. This army had a strength of between a moderate 30,000 and a fantastical 400,000 troops; the latter number is wildly inaccurate, as Heraclius' largest command had been 70,000 men during his war with the Sassanids, and the Byzantines also had to keep some forces in Italy, the Balkans, and the Caucasus in order to check the encroaching Lombards, Slavs, Avars, and Khazars. Heraclius, who was sick with edema, could not lead his Greek, Armenian, and Arab Christian army, which was instead divided into five columns led by the generals Vahan, Qanateer, Dairjan, Jabalah ibn al-Aiham, and Gregory. Dairjan, Jabalah, and Gregory would surround the Muslim forces around Emesa, while Qanateer's column would take Damascus and cut off Yazid ibn Abi Sufyan's Arab reinforcements from reaching Khalid ibn al-Walid and Abu Ubaidah's northern group. The army left Antioch in mid-June 636, but, a few days before the Byzantines' leading army reached Emesa, the Arabs' northern army withdrew, having been informed of the Byzantine plans from prisoners taken in a raid on Shaizar. Abu Ubaidah ordered his corps to fall back, and the Arabs retreated to Jabiya between the River Yarmouk to the south, Lake Tiberias to the west, and the desert to the east; Damascus was indefensible against the Romans. The Byzantines chased the Arabs, retaking Emesa and Damascus. In July of 636, the Muslim rearguard under Yazid fought against the Byzantine advance guard under Jabalah near Jabiya. The Arabs now understood that they might be attacked from Caesarea to the southwest, between Lake Tiberias and the River Yarmouk. Abu Ubaidah left Khalid in command of the rearguard and started repositioning his troops, engaging the Byzantines at the Battle of Yarmouk. In a six-day battle, the 140,000-strong Byzantine army was annihilated, with half of their number being killed and the remainder fleeing; the Muslims lost only a fifth of their number in a hard-fought, decisive victory.