The Byzantine Empire and the Sassanid Persian Empire had been archrivals for nearly four hundred years: ever since the Eastern Roman Empire split from the Western, and the Sassanids replaced the Parthians in the third century. In the early seventh century, the two old rivals fought a twenty year increasingly expensive and destructive war for control of the Near East and Egypt that in the end resulted in little change to the status quo, and left both empires exhausted. While both empires hammered on each other, the prophet Mohammad preached Islam in the deserts of Arabia, and just after his death, the Rashidun Caliphate was poised to expand beyond the Arabian Peninsula.

In 634, the Muslim Arabs exploded into the Levant (modern Israel, Jordan, Lebanon, and Syria), and quickly overran the Byzantine Christian lands. To the credit of both the Sassanids and the Byzantines, they recognized the threat that the Rashidun Caliphate represented. Byzantine Emperor Heraclius secured an alliance with the Persians, but the damage they did to each other was extensive and complete. Nonetheless, Heraclius concentrated his diverse armies into an overwhelming mailed fist, and in May 636 sought to crush the invading Muslims.

Khalid ibn al-Walid, the supremely competent, dedicated, and faithful Muslim battle commander, was outnumbered at least four to one, and fell back to the Yarmouk River, where if he was defeated, could easily seek refuge in the stronghold of Najd. Heraclius, whose army was wracked with logistics difficulties (the Byzantines never concentrated their armies for exactly this reason; although the bureaucracy was extensive, the effectiveness of the old Roman supply system was long gone), and bickering factions (ditto: the Slavs, Armenians, and Christian Arabs couldn’t get along), felt compelled to fight the Muslims on the ground of their choosing.

The battle began with Muslim warriors challenging various Byzantine commanders to single duels, but after losing several of his best soldiers, Heraclius attacked. For five days, the Byzantines pounded the Muslim lines, but the attacks were uncoordinated, and breakthroughs were never exploited because of the emperor’s cumbersome, complicated, and lethargic command system (one could say it was “Byzantine”… Ba Dum Tiss!). Furthermore, because of the feuding, Heraclius never committed his reserve to the fight: his cataphracts, the most powerful armored mounted force in the world, and hundreds of years ahead of the mounted knights of the West, whom sat watching the battle for most of six days. Conversely, Khalid’s mobile guard, comprised of the best warriors of Islam, consistently and routinely appeared at troubled spots on the line, stabilized it, and quickly moved off to another threatened area to repeat.

Nevertheless, Heraclius’ almost secured victory twice: On the third day, his army broke through the Muslim right which fell back to its camp. But there the defeated warriors were met by their camp followers. The enraged wives and servants used tent poles to beat the warriors back into the line. Not wishing to face the wrath of their women, the ashamed warriors counterattacked. With no follow up, the overextended Byzantines fell back. And again on the fourth day, when Byzantine horse archers (a direct result of experience fighting the Persians) dominated the Muslim Left. The fourth day of the battle would be forever remembered by Muslims as, “The Day of Lost Eyes” due to the uncanny accuracy of the Byzantines. The lack of follow through, and darkness, saved the Muslims.

On the fifth day, Heraclius, whose disorganized army by this point had thousands of casualties and low morale, asked for a truce. Khalid, sensing weakness declined. The next day, Khalid concentrated all of his cavalry in a decisive flank attack, and before his infantry broke, smashed the Byzantine Left in an unexpected charge.

Khalid ibn al-Walid’s stunning victory at the Battle of Yarmouk cemented Muslim control of the Levant for the next 450 years, and was the first Muslim conquest outside of Arabia. It would not be the last.