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If one were inclined to rank the most important battles in world history, the Battle of Yarmouk probably should be pretty high on the list. It eliminated, over the course of one 6-day battle, almost the entirety of the Byzantine military south of the Taurus Mountains, leaving Syria and the rest of the Levant (with Egypt waiting beyond that, and then the rest of North Africa) open to Arab conquest. Along with the Battle of Qadisiyah, which was fought in November of the same year and essentially destroyed the Sasanian (Persian) Empire, Yarmouk established the Islamic caliphate and thus helped fundamentally change the course of history.

According to the historical sources, Yarmouk’s result can be attributed to two factors that worked against the Byzantines despite their (somewhat, we can’t be sure how much) larger numbers and better equipment: intelligence (the military kind) and intelligence (the human brain kind). The Byzantine Emperor, Heraclius (d. 641), alarmed at several smaller Arab successes in Syria (including the capture of Damascus in 634), amassed a large army under the command of an Armenien general named Vahan, to try to pick off several much smaller, dispersed Arab forces in the Levant one by one. Unfortunately for Heraclius, the Arabs were able to find out about his counterattack (there’s the military intelligence) and adjust to it. Those dispersed armies were all recalled to join the main Arab army in the region at Jabiyah, under the nominal overall command of a man named Abu Ubaydah ibn al-Jarrar.

(As an aside, when I use the word “Arab” here and in other places when I’m talking about the early Islamic conquests, it’s shorthand. There were non-Arabs in the “Arab” armies—for that matter, there were Arabs in the Byzantine and Sasanian armies that the Arab armies were fighting.)

The sources suggest that the Byzantine army was somewhere in the neighborhood of 100,000 men, which would have been absolutely massive for that time, and that the combined Arab army was around 24,000 men, also very large. These numbers, which come largely from the Arab sources, are undoubtedly exaggerated to emphasize the great Arab victory against all odds—it stretches disbelief to think the plague-stricken, war-weary Byzantine Empire could’ve mustered that large an army at this point in its history even including allies. But I think it’s fair to say that the Arabs were outnumbered to some degree. Luckily for them, Abu Ubaydah had the good sense to defer most decisions to his second-in-command, Khalid ibn al-Walid, and Khalid’s nearly unmatched abilities as a general helped overcome the numerical disadvantage (there’s the human intelligence).

Khalid was already held in renown for his generalship after performing remarkably well in the Ridda Wars and in some earlier, smaller campaigns against both the Byzantines in Syria and the Sasanians in Iraq. He was held in such renown, in fact, that when the second Caliph, Umar, came to power, he removed Khalid from his own command and placed him under Abu Ubaydah. New emperors sometimes get nervous about having popular and successful generals out there being so popular and successful. Umar, as caliph, may have been threatened by Khalid’s growing popularity and/or his military prowess, but Abu Ubaydah, who was not caliph, didn’t care about any of that, and he wasn’t dumb enough to try to micro-manage a man who certainly seems to have been something of a military genius.

Arab and Byzantine troop movements, 635-636, leading up to Yarmouk (Arunreginald via Wikimedia Commons)

After a couple of weeks of skirmishes between the two armies, they wound up near the Yarmouk River in the modern Israel-Jordan-Syria border region. The Byzantines were biding time, waiting for the Persians (with whom they’d recently concluded an alliance) to put together an army that could attempt to retake Iraq and force the Arabs into a two-front war. But the strain of keeping such a large army in the field started to wear on the Byzantines, with supplies limited and the army’s various Greek, Armenian, and Christian Arab factions beginning to squabble among themselves. The Arabs also decided to force the issue. Khalid, we’re told, arranged his numerically smaller army in a large number of small units in order to make it look bigger, and the Byzantines felt obligated to attack before more Arab reinforcements could arrive.

For the first four days of the battle, the Byzantines pressed the offensive but the Arabs, though very hard-pressed at times, refused to break. At one point on day 2, sources relay that the Muslim left flank almost fell apart, but soldiers retreating toward their camp encountered their angry wives (yes, the army traveled with families in tow). Apparently—if this story is true—the soldiers were more afraid of their wives than of the opposing army, because they promptly returned to the battlefield and managed to hold out long enough for Khalid to shift troops over to support them. It’s also possible that at some point in the battle the Arabs set up a trap by hiding reserves in in their camp and drawing in the Byzantine cavalry. Finally, after a lull in the fighting on day 5, Khalid amassed his entire cavalry force for an offensive the next day. This was the decisive stroke. First they chased off the Byzantine cavalry, then they got around the Byzantine left flank and attacked from the rear.

Though we like to think of our modern world as more civilized than the world of the distant past, the fact is that most of the deadliest battles in human history are modern, because technology has made it easier and easier to kill large numbers of people. In pre-modern battles, the heaviest casualty figures are usually found in battles where one side gets hemmed in and then completely dissolves into a disorganized, chaotic retreat (the Battle of Cannae, where Hannibal’s men slaughtered anywhere between 2/3 and 7/8 of the opposing Roman army, is the classic case in point). Men are killed by the army chasing them, but also by their stampeding comrades and major obstacles in the way of their flight.

So it was here, where the Byzantine line of retreat was complicated by rivers and mountains and it was a relatively simple matter for the Arab cavalry to cut off the easiest paths of retreat to the north. It’s estimated that about half the Byzantine army (whatever size you think it was to begin with) wound up dying, and there are reports of men falling off of cliffs and drowning in rivers in addition to being cut down by the pursuing Arabs. The loss of the army was devastating for the Byzantines; the expense of building, equipping, and training a new one was massive and the empire wasn’t exactly flush with cash by this point. This is why Byzantine emperors were usually reluctant to commit their armies to pitched battles, because losing an army in this way was almost always worse than simply losing territory or even a major city. It wasn’t like the good old days, when a Roman defeat just meant that the empire had to tap a little deeper into its then-seemingly inexhaustible supply of manpower. The empire just couldn’t rebound from a major defeat the way it had back in those days.

Yarmouk’s outcome seems almost inevitable today with the benefit of hindsight. The Byzantine Empire had just been weakened by plague and a major war with the Persians and their empire was coming apart along ethnic and sectarian lines, while the Arabs were fresh, devoted, and seemingly better-led. But it must have been a shock to people who were alive when it happened. Yes, Arab armies had been successful in small clashes with Byzantine forces before this, but here they defeated a large (relatively, at least) Byzantine army in a pitched battle and, in doing so, signaled decisively that there was a new military power on the block.

Before Yarmouk, as far as the empire knew, these Arabs were just a big raiding party. If any Byzantines were aware of Muhammad, or Islam, or the imperial structures that had already started forming at Medina, it was merely in passing. But they certainly got the message. After Yarmouk, the Byzantines effectively holed themselves up in Anatolia and went on the defensive. It worked, too, more or less—a couple of Arab sieges of Constantinople notwithstanding, no Muslim army was able to take and permanently hold territory in Anatolia until the Seljuk Turks won the Battle of Manzikert in 1071. But the decision to retreat meant that the rest of the empire, from Syria to Egypt to the rest of North Africa, would pretty quickly be lost to the Arab conquerors. The hope in Constantinople was that, while the countryside was lost, the major cities of the region would be able to resist the Arab armies long enough for the empire to rebuild its shattered army. But it was only a hope, not a plan, and we know how things turned out.

What about Khalid b. al-Walid, the hero of this and many other battles for the Arabs? Umar, still upset about his general’s popularity, finally drummed him out of the army altogether in 638, and he died in 642. It’s believed that Umar grew to regret this decision, and when he was on his deathbed in 644 he’s supposed to have offered his regret that Khalid didn’t live long enough to succeed him as caliph.