Before the Battle of Yarmouk broke Byzantine military power south of the Taurus and Anti-Taurus Mountains pretty much for good, Caliphal forces had already taken the jewel of Roman Syria, Damascus, nearly two years earlier. How they did so may illustrate part of the reason why the Arab armies had such a relatively easy time conquering such a big chunk of the Byzantine Empire.
Today, of course, Damascus is a depleted shell of a city, hollowed out and brutalized by the Syrian civil war. But Damascus is one of the oldest continuously inhabited sites on the planet, with evidence of settlement as far back as the 9000s BCE and of urban settlement as far back as the second millennium BCE. Under Roman control from 64 BCE on, its importance as a marketplace for caravan goods from the east and south allowed it to prosper, and by the 7th century it was one of the most important cities in the entire world. However, its defenses were weakened, as were the defenses of the entire Byzantine Empire, by the fact that the empire had just come out (in 628) of a very destructive war with the Persian Sasanian Empire. The Sasanians had actually controlled Damascus from 613 through the end of the war, though since they wound up losing the war they had to return it to the Byzantines. The point is, Damascus, like the rest of the empire, was battle-weary and ripe for the picking from a military standpoint.
As the entry point for Arab trade goods heading for Roman markets, Damascus would have been the “big” city most familiar to well-traveled Arabs (i.e., traders). So it’s no wonder that it was a target for the Arabs from the first moment their armies began marching out of Arabia. The first Caliph, Abu Bakr, ordered an expedition under the famous general Khalid b. al-Walid to capture the city in April 634, but that attempt fizzled and Khalid went on his merry way. After spending a couple of months weakening Byzantine defenses elsewhere in the Levant, Khalid returned and laid siege to Damascus on August 21 (Abu Bakr died the following day). Actually, “siege” might be overstating things; Khalid’s men didn’t have any siege engines, so they surrounded the city and attempted to starve it into surrender, while also exploring any surreptitious means they might find to get inside its walls. The opposing armies seem to have been pretty evenly matched, with maybe a slight manpower advantage for the Arabs.
Khalid, being by almost any measure a true military genius, made sure to cover his bases by fortifying his supply routes back into northern Arabia and by sending out detachments of troops to watch for and, if possible, interdict any Byzantine relief forces. This paid off when one of these detachments spotted a column of Byzantine soldiers heading toward the city in the early part of September; Khalid led about half of his army out and routed the relief force. He got a bit lucky here that the garrison inside Damascus didn’t attempt a breakout against the weakened besieging army; they would have had a significant numeric advantage and could well have broken the siege if they’d made the attempt. The garrison did eventually attempt two breakouts (once it became clear that no Byzantine relief was going to arrive in time), first from the Gate of Thomas on the eastern side of the city and then from four different gates at once, but both of these were beaten back and cost the Byzantines thousands of men.
Now we get our glimpse (maybe, if the accounts are accurate) of the real problem facing the Byzantines: sectarianism. The sources relate that a Syrian priest approached Khalid one day to tell him that the Byzantines were planning a celebration for that night and that it might be a good time for the Arabs to try scaling the walls and attacking the city. If this is a real story and not a later invention of the Muslim chroniclers, here’s a citizen of Damascus selling his city out to the besieging enemy. Why? Well, part of the reason may have been that the priest was a Monophysite, a Christian who believes that Christ had only one nature (divine). “Official” Christianity, as decided at the Council of Chalcedon in 451, was Dyophysite, which means it held that Christ had two natures (a human one and a divine one). Compounding this sectarian issue was the fact that many of the people resident in Damascus were probably Arab, like the besiegers and, notably, not like the Greeks and Armenians in the city’s garrison. The story of the Monophysite priest may be accurate, or it may simply be some creative shorthand for the fact that much of the city’s population looked more favorably on the attackers than on their own defenders.
Maybe you’re wondering why this difference could possibly matter to anyone, let alone matter enough that a guy would be willing to subject his entire city to a bloody conquest because of it. Well, my advice would be to just accept that people back in the seventh century really cared about this kind of thing, and let it go. The official Church didn’t treat Monophysites very well, and most Christians in the Levant and Egypt were Monophysites (or, more accurately “Miaphysites,” which is more moderate than true Monophysitism but is close enough that Byzantine authorities usually just lumped the two together). To them, these Arabs with their nascent “Islam” were no worse than the Dyophysite Church, and maybe better, since at this point “Islam” was barely an idea, much less a religion, and conquest by the Arabs offered the possibility that everybody would be allowed to worship as he or she wished.
Anyway, whether he learned of the celebration from an inside source or just observed that a lot of guards seemed to be wandering off of the walls to go get drunk (you might get either story depending on whose account you read), Khalid was savvy enough to take advantage of the opportunity and order his soldiers to take their ropes and climb the city’s eastern walls. When he saw what was happening, the Byzantine commander, a fellow named Thomas, showed some serious quick thinking, crossed to the western side of the city, and promptly surrendered to the senior Arab officer there (Khalid’s deputy, Abu Ubaydah, who probably had no idea what Khalid was doing on the eastern wall). Why? Because cities that surrendered to the Arabs peacefully tended to fare much better than cities that resisted, and sure enough Abu Ubaydah’s terms spared the lives and property of the Damascenes in return for the payment of tribute to the Arabs.
When Khalid found out that Abu Ubaydah had cut a deal with Thomas he was livid, but the Arabs recognized that to abrogate the surrender agreement would send a message to other Byzantine cities that they shouldn’t surrender when besieged, because the Arabs couldn’t be trusted to abide by their deals. So the terms of the surrender were honored and the city was spared any severe violence. This paid off later, when the unsacked, still thriving Damascus became the capital of the Umayyad Caliphate and reached new heights of wealth and importance. For his trouble, Khalid was recalled to Medina by the new Caliph, Umar, and stripped of his command (follow that link there if you want to know why). He would bounce back at Yarmouk, but he was never on good terms with Umar as he had been with Abu Bakr.