The Byzantines and their Persian enemies either ignored or were not fully informed of the upheavals in the desert, of the years of battles and skirmishes between the Muslims and the much more powerful tribes that surrounded them. But at midsummer 633, a large desert army suddenly appeared before the Persian walled town of Hira, on the desert frontier south of the Euphrates. At the time of the Muslim advance, Persia was weak, first from years of conflict with the Byzantines, and second from civil war among the ruling dynasty. Within a matter of months, fast-moving Muslim columns had progressed northward, destroying isolated Persian garrisons, killing those who opposed them, by crucifixion or with the sword. Their enemies eliminated, they enlisted converts to Islam from among the local peasants, and passed on. They levied tribute rather than settling the lands they had conquered.
Thus, like rural guerrillas throughout history, the Arab Muslims seized control of borderlands. The desert, the element in which they were supreme, was like a sea that washed the shoreline of the settled lands, in Syria and Palestine, and along the fertile plain between the Tigris and the Euphrates. They moved with astonishing speed, wrong-footing their static Persian and Byzantine enemies. A Muslim army fighting east of the great rivers of Iraq suddenly changed direction, marched southwest around the Syrian desert, then north toward Palmyra. At one stage, it had to pass through a completely arid region without oases or water holes. Here the desert lore of the tribesmen came into play, as they searched for desert plants that indicated water just below the surface. They had also made their camels drink to fill their internal water bladders at the last oasis. Then each day they killed four of them, and cut out the sacs still full of water to provide liquid for the few precious horses.
At the same time another Arab force, at least 3,000 strong, but perhaps numbering as many as 9,000 men, huge by the standards of desert campaigning, was pushing northwest from the Arabian peninsula toward the Mediterranean coast. The Byzantine emperor Heraclius, a veteran of decades of war against the Persians, was now aware of the danger posed by the desert Arabs. He dispatched a field army from Homs in Syria to intercept the Muslims advancing north from Beersheba. This Muslim army had already tasted victory, sweeping aside the local forces commanded by the governor of Gaza. A Syriac chronicle written sometime after the event described the outcome of the encounter. “The Byzantines fled and abandoned the commander … and the Arabs slew him. Some 4,000 poor villagers from Palestine were killed, Christians, Jews and Samaritans, and the Arabs destroyed the palace completely.” Heraclius had sent the bulk of his soldiers south along the flat coastal strip. The Byzantine column passed the town of Ramleh, heading toward the Arab army, reported as being still near to Gaza. A force of this size moved slowly, keeping close to the sea so that it could be supplied from the Byzantine fleet cruising off the coast. These were not just poorly trained local conscripts but included a nucleus of heavily mailed infantry, horse archers, and armored heavy cavalry—the cataphracts.
At some point south of Ramleh the Byzantines were ambushed by the Arabs—the Muslim army had advanced farther north than Heraclius or his commanders had anticipated. The exact location of the ensuing battle of Ajnadain is uncertain. But the outcome, on July 31, 634, was an annihilating victory for the Muslims. The Byzantine force was following the track south, giving a wide berth to the Jabal Nablus (Judean hills). The land was flat, not the broken ground or ravines that favored irregular fighters. The only advantage that the Muslims possessed was their speed of maneuver and their reckless zeal. All day long the few Arab horsemen darted at the enemy, tempting them into pursuit, and drawing the fire from their bowmen. Their leader, Khalid Al-Walid, had to remind his men to “reserve yourself until the evening. It was in the evening that the Prophet was accustomed to vanquish.” But once it had broken the enemy ranks in the late afternoon, the Byzantine army’s advantages of larger numbers and better equipment vanished. The Muslims could gut their opponents’ horses with their knives and spears, and mob the heavily armored infantry. Meanwhile, with the light failing the Byzantine archers had no clear targets.
As they slashed and stabbed, cutting off the heads and limbs of their opposites, shouting their tribal battle cries, the Arabs’ assault destroyed their enemy’s cohesion. The bewildered Byzantine troops ceased to obey orders, milling about as they tried to fight back against their gadfly foe. Once the Byzantines’ momentum had slackened and they began to retreat, the Arabs turned this attempt at maneuver into a rout. The Byzantine troops fled north, pursued by the Arabs for a while, until the latter returned to strip the bodies of the fallen soldiers of their arms and equipment. Tribesmen who had been armed with nothing more than a spear now acquired a horse and a sword. Buoyed up by victory, the Muslims moved against the cities and towns. Jerusalem shut its gates, but outside the walls, according to the patriarch of Jerusalem, the tribesmen “plunder cities, despoil the fields, burn the villages, despoil holy monasteries.” In reality, the Muslims were more concerned with consolidating their authority. Far from mindlessly ravaging the land, they decreed that the Christian inhabitants should pay an annual poll tax of one dinar, and hand over a percentage of their crops each year.
The disaster at Ajnadain was wholly unexpected and threw all the plans of the emperor Heraclius into complete disarray. He had beaten so many powerful adversaries that it seemed incredible that he could now be humbled by a rabble of Arab tribesmen. However, the reports he received showed that these desert raiders, clad in coarse cloth or even skins, armed with a spear and a long knife, were as dangerous as the skilled Persian soldiers whom he had fought against for half his life. He quickly withdrew from Homs, which might be threatened by Arab raiders, north to the safety of Antioch, and set about rebuilding his army. He paid the Christian Arab tribes that had once garrisoned the frontier to fight for him again. He summoned the redoubtable Armenians from over the mountains in Anatolia. Meanwhile, the Muslims concentrated their forces in a well-chosen position close to the town of Deraa, on the river Yarmouk. The Byzantine armies, moving down from the north, could approach them only from the front. The land to their left was fissured by deep ravines, while to the right was a lava field, the residue of an extinct volcano. Small groups of warriors could work their way across these badlands, but no organized army would choose to fight in such conditions. The Byzantines set up a strongly fortified camp just to the north of the Arab position. This blocked the Muslims from advancing into Syria, and if they chose to attack, the Byzantine commanders were confident that their wild and savage enemies would die in their thousands before the well-built ramparts.
However, the Muslims resorted to the classic maneuver of desert war: a mobbing attack from all sides. Small groups scrambled through the ravines of the Yarmouk, and hid behind the Byzantine position. Others, on horseback or on camels, crossed the gray lava shale, and began to raid the Byzantine lines of communication back toward Syria. The two armies confronted each other for over four months, and the Byzantine forces ran short of food and other supplies. Their lifeline was the road to the ports of Tyre and Sidon, but all the supplies had to be carried across the bridge over the Yarmouk, only a few miles from the great encampment. In the middle of August, Muslims hiding in the ravines emerged and captured the lightly guarded bridge. This cut the Byzantines off from the coast, with their only line of retreat through the desert to Damascus. Early on August 20, 636, the wind began to blow off the desert, picking up sand and dust, with the sky turning the color of dull bronze. As the day went on, it became darker and darker, the air so thick with sand that it was hard to breathe and impossible to see more than a few yards ahead. Struggling with the elements, the Byzantines suddenly faced the full weight of an Arab attack. It was late in the afternoon, and by the time night fell, around six, the Muslims had broken through the lines at many points. All attempts at a coordinated defense fell apart, and through the night the Byzantine troops were slaughtered where they stood. By dawn their dead lay in huge heaps on the ground, with the Arabs stripping the corpses of every object they could use. Then they abandoned the battleground to the crows and other scavengers.
Only a few thousand of the Byzantine army managed to slip away in small groups to take the news of the devastating defeat to Heraclius in Antioch. After the cities of the region had fallen one by one to the Muslims, the emperor eventually concluded that the whole Levant, from the border with Egypt to the Taurus Mountains at the edge of Anatolia, was untenable. His remaining forces withdrew through the narrow pass called the Cilician Gates, after destroying all the towns and villages for miles around. From then onward, the frontier of the Byzantine Empire lay on the high plateau of Anatolia, and not in the rich and fertile lands of the Levant. The emperor relinquished Jerusalem as well as the Christian Arabs who had formed much of his support in the region. However, the image of sudden and catastrophic abandonment is exaggerated. The Levant had been fought over by the Byzantines and the Persians since the days of Justinian. Heraclius and the Byzantines had regained full control of the Holy Land from the Persians barely ten years before the arrival of the Muslims. Moreover, many of the Christians in the region were regarded as schismatics and heretics by the Orthodox authorities in Constantinople, who had oppressed them remorselessly. This is one reason why some of the Christian accounts present conflicting views of the Muslim conquest. It was the Orthodox who usually anathematized the Muslims most fiercely. They were not universally regarded irreconcilably as enemies by all the Christian communities.