By Pat Lowinger
In Part One we examined several of the factors which had severely weakened the Byzantines in the century prior to the Arab invasion of Roman Syria. These factors included a series of devastating plagues, severe economic stress and a war-weary military.
The Muslim Invasion of Syria (April, 634 CE)
The grand military strategy of the Byzantine Empire was designed to be primarily defensive. This defensive plan relied upon the establishment and maintenance of border garrisons along the frontiers of the empire. These border forts were designed to defend strategic strong points and provide vital information regarding enemy troop movements. In theory, as well as previous practice, an enemy that overwhelmed or bypassed one or more of these garrisons would have given the Byzantine military the time necessary to mobilize field forces stationed in the imperial provinces. The prevailing military mindset of the empire prior to this period was that any attack which penetrated into the interior of the empire would eventually be repulsed. Thus, any territories lost to an enemy would be temporary, regionally limited and eventually restored to imperial control.
After having marshaled four separate armies, Abu Bakr attacked a weakly defended portion of the Byzantine’s border just north of the Gulf of Aqaba. Abu Bakr assigned command of an army to each of four generals: Amr ibn al-A’as, Yazid ibn Abu Sufyan, Shurahbil bin Hassana and Abu Ubaidah ibn al-Jarrah. Soon after crossing the border, the four armies separated and each moved rapidly to secure strategic objectives. After defeating the Byzantine’s Ghassanid allies in two consecutive battles, Shurhabil moved towards the key city of Bosra. The first weeks of the Arab invasion caught the Byzantines largely off guard. The initial Byzantine response could be best described as chaotic; some garrisons abandoned their posts while those garrisons which remained in place were simply bypassed as the Arab forces rapidly advanced into the province. In northern Syria, the Emperor Heraclius began marshalling his own forces. Abu Bakr responded to the build up of Byzantine forces by recalling his most skilled general, Khalid ibn al-Walid, from the offensive in Persia to personally oversee the campaign in Syria. By early June, Shurhabil had encircled Bosra and placed the city under siege. By mid July, the Byzantine counteroffensive began to move southwards under the command of the Emperor’s brother, Theodore.
After more than a month-long siege, Bosra fell to Shurhabil and Khalid’s combined forces. Khalid then moved his army toward the lead elements of the Byzantines near Ajnadayn. In late July 634 CE, the armies met. Military Historian Michael Decker has stated that the strength of the two armies was relatively equal and consisted of approximately 20,000 men each. One of the most commonly utilized historical sources for information regarding the Syrian campaign is Al-Waqidi’s well-known account, but is generally considered to lack credibility in regards to the respective sizes of opposing forces and/or causality figures. While Al-Waqidi’s record indicates 50,000 Byzantines were slain with only the loss of 575 Arabs, Decker asserts that heavy casualties were suffered on both sides.  Arab casualties included the loss of several prominent nobles, which appears to support Decker’s hypothesis. What is clear is that the defeated Byzantine army was forced to retreat north of the heavily fortified city of Damascus. On August 21, 634 CE, Arab forces surrounded the city. Despite two attempts by the defenders to break the siege, Khalid’s troops captured the city on September 19th.
In 635 CE, the Emperor Heraclius placed the combined forces of the empire under the command of the Imperial Treasurer Theodore Trithyrius. Despite his long and dedicated service to the state, Trithyrius had little or no military experience. The appointment caused derision among several prominent and experienced Byzantine generals. The army that numbered between 100,000 – 150,000 infantry and cavalry had been charged with expelling the Arabs from southern Syria and recapturing Bosra. In May 636 CE, the army moved southwards towards the Yarmouk River in pursuit of Arab forces. Khalid ibn al-Walid’s force was estimated to have numbered no more than 40,000. Despite reported warnings from their Christian Arab allies, Trithyrius ordered the army to pursue the highly mobile Muslim forces. For several days, Khalid’s forces lured the slow and ponderous Byzantine army into increasingly rugged terrain. On August 15, 636 CE, having carefully positioned his forces, Khalid’s light Arab cavalry began their attack. Upon realizing that he had foolishly led the army into a well laid trap, Trithyrius transferred command of the army to his subordinate, Vahan. The battle of Yarmouk lasted for 6 days and was largely characterized by the rapid hit and run tactics employed by Khalid’s forces. On the 6th day of the battle, Khalid’s cavalry were finally successful in driving off the Byzantines’ protective cavalry screen. With their flanks now exposed, the large formations of Byzantine infantry were surrounded and largely annihilated. Casualties among the Byzantines are estimated to have exceeded 50%. Historian George Ostrogorsky stated the following, “They [Khalid’s forces] completely routed the Byzantine army at the famous battle of Jarmuk [Yarmouk] on 20 August 636, and thus Byzantine resistance was broken and the fate of Syria decided.”
Following the defeat at Yarmouk, Antioch, the provincial capital of Syria capitulated to the advancing Arab forces. Al-Waqidi recorded the surrender of Antioch as, “That night the city leaders gathered around the Patriarch and said, ‘Go to the Arabs and obtain whatever terms you can.’ ” Several smaller cities in the region also surrendered to the Arabs with no opposition being offered. A notable exception occurred at Aleppo. In August 637 CE, Arab forces surrounded the fortified city of Aleppo. The siege lasted for three months. During that time several violent skirmishes were fought between Arab and Byzantine forces. Ultimately, the garrison forces were unable to break the siege and accepted terms for their departure from the city under a flag of truce.
In Palestine, Arab forces began a protracted siege of Jerusalem in November 636 CE. Defense of the city was placed under the command of the city’s Patriarch, Sophronius. The city’s considerable defenses meant that any direct assault would prove costly. The Muslim forces instead surrounded the city in order to starve out the defenders. In April of 637 CE, the Byzantines resolved themselves to the idea that no relief from the Empire was forthcoming and sought terms with the forces of Caliph Umar ibn Al-Khattah. Under the terms of the agreement, all Christians and Jews within the city would be allowed to keep and practice their own faiths. The city’s garrison and officials were allowed to depart by sea. At Caesarea, Heraclius’ son Constantine III fled by ship rather than risk possible capture as Arab forces advanced towards the city. The remaining garrison and civilians surrendered to Amr bin al-As in 640 CE after a protracted siege.
The century before the Arab conquest of Roman Syria was marked with a steady decline in the wealth and stability of the empire. The recurrent plagues that began during the reign of Justinian I decimated the population and economy of the empire. These factors were further compounded by the 2 ½ decades long war between the Sassanids and Byzantines. While ultimately victorious, the empire’s treasury and military were exhausted.
There can be little doubt as to the quality and dedication of the Arab forces which fought for control of Syria and Palestine. With that said, it is important to remember the systemic failures within the Byzantine military facilitated the Arabs’ unprecedented success. The static defense offered by the empire’s border garrisons, which had long protected the empire, were easily circumvented by the rapidly advancing and highly mobile Arab troops, thus largely negating the advanced warning they had previously offered. Poor and inexperienced leadership exacerbated these issues and in all likelihood was responsible for the Byzantines’ disastrous defeat at Yarmouk. The Byzantines were not without their own insights into the Arab-nomad style of warfare, having themselves numerous Christian Arabs who had long been trusted allies. The ultimately failure of the Byzantine military was exacerbated by the inability of the Emperor Heraclius and his commanders to adjust their tactics as necessary to meet the fast, hard-hitting and often evasive maneuvers of the invading Muslim Arabs.
Bibliography and Citations:
Decker, Michael. The Byzantine Art of War. Yardley, PA: Westholme, 2013.
Goldschmidt, Arthur, and Lawrence Davidson. A Concise History of the Middle East. Boulder, CO: Westview Press, 2006.
Haldon, John F. The Byzantine Wars. Gloucestershire: History Press, 2009.
Haldon, John F. Warfare, State and Society in the Byzantine World, 565-1204. London: UCL Press, 1999.
Mango, Cyril A. The Oxford History of Byzantium. Oxford: Oxford University Press, 2002.
Mazor, Amir. “The Kitab Futuh Al-sham of Al-Qudami.” Der Islam 84, no. 1 (2007): 17-45.
Ostrogorski, Georgije. History of the Byzantine State. New Brunswick, NJ: Rutgers University Press, 1969.
Rosen, William. Justinian’s Flea: The First Great Plague, and the End of the Roman Empire. New York, NY: Penguin Books, 2008.
Theophanes, The Chronicle of Theophanes: An English Translation of Anni Mundi 6095-6305 (A.D. 602-813). Translated by Harry Turtledove. Philadelphia: University of Pennsylvania Press, 1982.
Tsiamis, Costas, Effie Poulakou-Rebelakou, and Spyros Marketos. “Earthquakes and Plagues During Byzantine Times.” Acta Medico-Historica Adriatica 11, no. 1 (2013): 55-64.
Wāqidī, Muḥammad Ibn ʻUmar, The Islâmic Conquest of Syria: a Translation of Futûh̲ushâm: The Inspiring History of the S̲ah̲abâh’s Conquest of Syria. Translated by Sulayman al-Kindi. London: Ta-Ha, 2005.
 John F. Haldon, Warfare, State and Society in the Byzantine World, 565-1204 (London: UCL Press, 1999), 43-46.
 Ibid., 67-71.
 Muḥammad Ibn ʻUmar Wāqidī, The Islâmic Conquest of Syria: A Translation of Futûh̲ushâm: The Inspiring History of the S̲ah̲abâh’s Conquest of Syria, trans. Sulayman al-Kindi (London: Ta-Ha, 2005), 12.
 Ibid., 44-45.
 Ibid., 52.
 Theophanes, Chronicles, 38-39.
 Michael Decker, The Byzantine Art of War (Yardley, PA: Westholme, 2013), 20-21.
 Ibid., 21.
 Decker, The Byzantine Art of War, 21-22.
 Haldon, The Byzantine Wars, 58-60.
 Ibid., 59-60.
 Al-Wāqidī, The Islâmic Conquest of Syria, 272-274.
 Haldon, The Byzantine Wars, 57-58.
Haldon, The Byzantine Wars, 62-64.
 Ostrogorski, History of the Byzantine State, 111.
 Ibid., 111.
 Al-Waqidi, The Islâmic Conquest of Syria, 522.
 Ibid., 408-415.
 Theophanes, Chronicles, 38-39.
 Al-Waqidi, The Islâmic Conquest of Syria,
 Ibid., 575.