August 20, 2021 marks 1,385 years since the Battle of the Yarmuk, a historic battle between the Byzantine Empire and the Rashidun Caliphate in one of the first major clashes between the Islamic world and Christiandom that saw the beginning of Islamic expansion out of the Arabian peninsula and the start of Islamic rule over the Levant.
The historical context for this battle comes following the death of the Prophet Muhammad in 632 CE, four years earlier, after he and his armies succeeded in conquering the Arabian peninsula and spreading Islam. He was succeeded by his father-in-law Abu Bakr, who became the first caliph, who was later succeeded just two years later by the second caliph Umar.
The new caliph was eager to follow up on his predecessor's expansion into Iraq and Syria. However, he soon came into conflict with the Byzantine Empire, with Emperor Heraclius — who had only recently conquered lands in the Levant and Syria from the Sassanid Empire (Persia) and wished to protect his gains and launch a counterattack.
The two sides would rally their armies and eventually meet south of the Yarmuk River, a major tributary of the Jordan River, southeast of the Golan and east of the Kinneret in what is now the border area between Jordan and Syria.
The exact sizes of the armies is a subject of considerable debate, with multiple primary and contemporary sources (all of whom were Arab, as the sole Byzantine source came a century after the battle itself) as well as modern estimates. However, it is widely accepted that the Byzantine army vastly outnumbered the Arab forces, with modern estimates saying the Byzantine army numbered between 80,000 to 150,000 compared to 15,000-40,000 for the Rashudin army.
However, the Arabs had an advantage in their position. The Rashudin commander, Khalid ibn al-Walid, had pulled his forces to an area that was ideally suited for cavalry and had established a strong defensive position. It was here that he organized the army into 36 infantry regiments along with four cavalry regiments and elite mobile guard cavalry in reserve. The infantry utilized the highly defensive Tabi'a formation, with the center and left and right flanks assigned cavalry to use as a counterattack. And at the rear was the mobile guard cavalry itself.
Their opponents had kept the regular heavy cavalry equally among its four units, each of whom kept them as a reserve in the rear with infantry at the front. As noted by historian David Nicolle, the Byzantine forces likely made use of the tortoise formation (testudo), a Greco-Roman formation where troops stood tightly together, shields held high to completely protect against arrows. Despite this, however, the Byzantines had been wary of a massive decisive battle, something they had tried to avoid for hundreds of years as the empire struggled with the logistics of maintaining a supply line. In fact, Emperor Heraclius had ordered the Byzantine commander Vahan to not engage in a battle until every other avenue was exhausted, though it may also have been due to waiting for newfound Sassanid allies to be ready for an offensive against Arab lands in Iraq.
These factors, along with a number of other issues, sparked tensions within the army, who now had no choice but to engage in a decisive battle, with infighting among the leadership resulting in even worse planning.
The resulting battle lasted six days. Throughout the battle, Walid made keen use of the mobile guard cavalry in several instances, inflicting heavy damage on the Byzantines in coordination with his infantry. Throughout the first few days, the Arabs had stayed defensive, but mounted a major offensive on the final day, utilizing a massive cavalry maneuver to strike at the flanks while the infantry charged at the center, but made use of the mobile guard's mobility to wheel and route the Byzantine cavalry, decisively defeating the far larger foe.
The battle was an absolute disaster for the Byzantines, and later analysis pointed out numerous flaws in their tactics throughout the battle, such as not taking advantage of their numerical superiority and not utilizing their cavalry.
By contrast, the Arab army managed to defeat a far superior foe, and Walid's keen understanding of his cavalry and taking advantage of their mobility has him in the conversation as one of the best cavalry commanders in history.
Ultimately, around 45% of the Byzantine army was killed, according to modern sources, with primary sources placing that figure as high as 120,000. By contrast, the Arabs only suffered around 30,000 casualties.
The battle signified a major step forward in the Muslim conquest of the Levant, with this victory paving the way for the capture of Jerusalem the following year.
Of note is also the view of the local Jews who lived in the Levant at the time. Though they played no role in the battle itself, sources indicate that they vastly favored the Muslims compared to the Christian Byzantines.
According to the 9th century Islamic historical text Futuh al-Buldan regarding the Arab conquest of Emesa, now known as the city of Homs in Syria, the local Jews shouted "We swear by the Torah, no governor of Heraclius shall enter the city... unless we are first vanquished and exhausted!" The Jews also reportedly celebrated with music and dancing when the Arabs won.
The Muslim conquests would only continue to expand throughout the world, and eventually forming one of the largest empires in history. It also saw the continued decline of both the Byzantines and the Sassanids, who were defeated at the Battle of Qadisiyyah just three months later and paved the way for the Sassanid empire's collapse just years later.
The Rashidun Caliphate would later collapse into civil war. However, the conquests in the Levant made sure the region stayed under Muslim rule, with nearby Damascus becoming the center of its successor, the Umayyad Caliphate.
But arguably most crucially of all, the battle marked the beginning of the many wars between the Islamic world and Christiandom, which would continue over the next millennia as Christian forces attempted to take the Holy Land and parts of North Africa while the Muslims continued to expand, eventually conquering Spain and reaching as far into Europe as the gates of Vienna.