The death of Muhammad in 632 AD was but a bump in the Islamic wars of expansion and consolidation which he had begun ten years earlier. Under his immediate successor, Abu Bakr, the Arabs more or less completed the unification of Arabia and much of what is now Jordan. However, there was a problem with the situation: the Islamic territories had no further room to expand. To the west, south and east lay seas or oceans; to the northeast lay the Sassanid Empire, and to the northwest the Byzantine Empire loomed. Either empire could, it was likely, crush their young neighbour in most circumstances. However, Kushrau II and Heraclius had just completed a ruinous war which seriously weakened both kingdoms. With their economies ruined and armies battered, they were ripe for outside invasion precisely as the Arab armies were approaching their borders.

By 636, the Arab armies were skirmishing with the Sassanid frontier garrisons. A terrible defeat at the Battle of the Bridge in 634, probably near Kufa, caused them to seek a weaker frontier to probe at. One was found in Syria. In 634, Khalid bin al-Walid, one of the greatest of the Arab generals, entered Byzantine territory by crossing a desert expanse from Palmyra to Syria with a small force of perhaps eight hundred men. (The crossing itself is one of those things which fascinates scholars of military history, as the desert in question was considered impassable at the time.) In a six-day march, the desert was crossed and Khalid's men struck into Syria, routing the Ghassanids in the area on Easter, defeating local Byzantine garrisons at Ajnadayn and Pella, gathering a larger army through more conventional march routes, and generally making a nuisance of themselves.

With the fall of Damascus, the emperor Heraclius finally decided to react. Gathering a large, polyglot army of Greeks, Armenians and others, the emperor advanced south to confront the Muslim invasion near the river Yarmuk, on what's now the border between Jordan and Syria. They confronted an Arab army under Khalid's command, and the battle began.

The Battle ofYarmuk presents both opportunities and problems for historians. It is one of the only battles - okay, the only battle - of the early Islamic period that is documented well enough to reconstruct in something resembling detail. However, problems abound in the primary sources, due to the tendency of Arab historians at the time to make much use of hyperbole and exaggeration to make their victories - impressive as they are - seem even larger. As an example, one such historian, Al-Baladhuri, claims that some 12,000 Muslims fought 200,000 Byzantines, who were so terrified at their presence that Heraclius had to chain his soldiers together to keep them from fleeing. Obviously this was not the case, and it is far more likely that a Muslim army of some 20,000 men was facing a Byzantine force of 20-25,000, possibly 30,000 at the outside.

The lineup of the two armies was fairly standard. Each had its centre, left, and right wings arrayed over a line perhaps two miles long.* The initial phase of the battle was a Byzantine general advance across the Wadi'l Ruqqad. The Muslim army under Khalid cautiously pulled back at first, possibly due to their overextending their own line of battle. The battle began in earnest with the Byzantine left wing assaulting the Arab right, which used a feigned retreat to draw them further ahead than they should have gone. As a result, some of Khalid's cavalry managed to drive a wedge between the attacking Byzantines and the rest of their army. In the ensuing melee, the Arabs siezed the one bridge across the wadi, cutting off the Byzantine retreat, and proceeded to surround and absorb the Byzantine left wing. With the loss of roughly a quarter of their army in one blow, Byzantine resistance quickly crumbled. The cohesion of Heraclius' army collapsed; with their backs to the steep banks of the wadi (the opposite bank of which was now held by Khalid's forces anyway) and surrounded on all sides, the panicked Byzantine army was largely destroyed.

One of the unusual things about the Battle of Yarmuk was its relative mundanity. Many of history's battles are remembered for some particular characteristic, whether it be a spectacular strategem like at the battles of Cannae or Austerlitz; a stroke of dumb luck as at Marathon or Midway; or instances of particular heroism or daring as at Thermopylae or Balaclava. Aside from the significance of the victory itself, Yarmuk was a fairly ordinary battle. The two armies used fairly typical forces of spearmen, swordsmen, archers and cavalry, lined them up in a standard manner, and fought a conventional battle in which Khalid emphatically came out on top. Khalid's other campaigns show that he is a military genius, but in this case the credit seems to fall as much to his soldiers as to his generalship. The Muslim army, simply put, outfought the Byzantines mano a mano.

The larger significance of the battle is that it essentially annihilated Byzantine power south of Asia Minor. The destruction of Heraclius' army - the emperor himself survived the battle if he was even on the field - allowed the Muslim armies to rampage through Syria, Palestine and Egypt, all of which had fallen by roughly 642. With the Byzantine emperor hiding on the far side of the Taurus Mountains, the road had been opened for Islamic expansion into northern Africa, which would eventually take the caliphates to Spain and beyond. On top of the rout at Yarmuk, another Arab army under command of Sa'd bin Abi Waqqas fought the Sassanids again at Qadisiya - at roughly the same time as the Battle of Yarmuk - and crushingly defeated them as well, leaving the way open for the Battle of Nihavand eight years later.

In the space of a few short months, much of the military power of the two great empires of the Middle East lay in ruins. Medina had broken through a wall separating Arabia from the rest of the world. The stage was set for one of the most astonishing military expansions in history.

* - My use of the terms "left" and "right" are from the point of view of the Arab army. The Arabs' left wing faced the Byzantines' right, and vice versa.

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