The Second Arab Siege of Constantinople (717–718)

The Second Arab Siege of Constantinople was a thirteen-month attempt by the Umayyad Caliphate to capture the Byzantine capital, lasting from August 717 to August 718. Led by the general Maslama ibn Abd al-Malik, a massive Arab army and fleet blockaded the city by land and sea, only to be broken by Emperor Leo III’s use of Greek Fire, a catastrophic winter, and a Byzantine alliance with Bulgaria. The failed siege is widely considered one of history’s turning points, halting the westward expansion of the early Islamic caliphates into southeastern Europe.

A city the Caliphate could not ignore

By the early 8th century, the Umayyad Caliphate was the largest empire the world had seen since Rome at its height, stretching from the Iberian Peninsula to the borders of India. Constantinople was the one prize that had eluded it. An Arab fleet had already tried and failed to take the city once before, in a long siege between 674 and 678, during which Byzantine engineers first unveiled a terrifying new weapon: Greek Fire. That failure had not discouraged Damascus. If anything, it sharpened the ambition. Constantinople sat at the hinge between Asia and Europe, controlling the straits that linked the Black Sea to the Mediterranean. Whoever held it did not simply own a city — they held the gate between two continents.

By 716, the Caliph Suleiman ibn Abd al-Malik decided to finish what his predecessors had started. He entrusted the campaign to his brother, Maslama ibn Abd al-Malik, one of the most experienced generals in the Umayyad state, and began assembling what medieval chroniclers describe as an extraordinary force: armies numbering in the tens of thousands and a fleet said to run into the hundreds, if not over a thousand, ships. Modern historians treat the largest of these figures — some medieval sources claim as many as 1,800 ships and 120,000 men — with real skepticism, since chroniclers on both sides had every incentive to inflate the scale of the threat Byzantium had overcome. What is not in doubt is that this was the single largest and most determined effort the Caliphate would ever make against the Byzantine capital, combining a land army advancing through Anatolia with a massive naval force meant to blockade the city by sea.

A new emperor, weeks before the storm

The timing could not have been more precarious for Byzantium. In March 717, just months before the siege began, a general named Leo the Isaurian seized the imperial throne from the short-lived and ineffective Theodosios III. Leo — who would become known to history as Leo III — inherited an empire that had spent the previous two decades in near-constant political chaos, cycling through emperors, coups, and civil wars while Arab raids pushed deeper into Byzantine Anatolia almost every year. It would have been reasonable to expect the empire to collapse under the pressure of a full Umayyad invasion arriving on the heels of yet another change of ruler.

Instead, Leo proved to be exactly the kind of emperor Byzantium needed at that exact moment: a hardened soldier from the eastern frontier who understood the Arab armies from years of fighting them directly, and a shrewd political operator who spent his first months on the throne strengthening the city’s defenses, securing grain supplies, and — critically — reaching out to potential allies north of the Danube. His timing, whether by skill or sheer luck, meant Constantinople faced the largest siege in its history under a ruler who had spent his entire career preparing, without knowing it, for precisely this fight.

The siege begins

Maslama’s land army crossed into Byzantine Anatolia in the summer of 717 and advanced largely unopposed toward the Bosporus, crossing into Europe near Abydos and marching on Constantinople from the landward, western side. By late August, the Arab army had reached the city and begun constructing siege lines, digging a double ditch and wall to protect their own camp both from Byzantine sorties and from any relief force. But Constantinople was not a city that could be starved out quickly, or overrun by a land army alone. Its famous Theodosian Walls — a system of double fortifications stretching across the entire landward approach to the city — had already turned back every land assault attempted against it for nearly three centuries, and would continue to do so for another seven hundred years, until 1453 – the Crusaders sacked the city through the sea walls in 1204.

The real Arab strategy depended on the fleet. Maslama’s plan was to combine the land blockade with a total naval blockade of the Bosporus and the Sea of Marmara, cutting Constantinople off from all seaborne supply and reinforcement, and slowly starving a city of several hundred thousand people into submission. In early September 717, the Umayyad fleet sailed up to the city and attempted to force its way into the waters directly beneath the walls.

It sailed into a trap.

Greek Fire and the Battle of the Bosporus

Leo III had prepared for exactly this moment. As the Arab ships advanced, Byzantine dromons equipped with siphons of Greek Fire moved out to meet them. Greek Fire — an incendiary substance, likely petroleum-based, that could be projected from shipborne siphons and that famously continued to burn even on the surface of water — had already proven itself against Arab fleets during the first siege forty years earlier, and it worked again with devastating effect. Panic spread through the tightly packed Umayyad ships as burning liquid clung to hulls, rigging, and men, and entire vessels were destroyed within the straits. Whatever the precise scale of the destruction, the psychological effect was total: from that point on, the Arab fleet largely avoided directly engaging the Byzantine navy in the straits, choosing instead to anchor at a safer distance and rely on the land blockade — which meant the total naval stranglehold Maslama had planned for never fully materialized. Constantinople, though under siege, was never entirely cut off from the sea.

An ally from the North

As autumn turned to winter, Leo’s diplomacy paid off in a way that would prove decisive. Khan Tervel of Bulgaria — ruler of a state that had, only a few years earlier, been a dangerous rival to Byzantium itself — chose to intervene on the empire’s behalf. Bulgarian forces struck the Arab army’s rear and its foraging parties north of Constantinople, inflicting what several chronicles describe as extremely heavy casualties, reportedly tens of thousands of dead among the besiegers. Whatever the true figure, the effect was to squeeze Maslama’s army from an unexpected direction at the worst possible time of year, cutting further into the food and supplies it needed to survive the winter and denying it freedom of movement in the countryside around the city.

The winter that broke an army

If Greek Fire and Bulgarian spears wounded the siege, the winter of 717–718 very nearly finished it outright. Byzantine and later Arab chroniclers alike describe one of the harshest winters in living memory, with snow lying on the ground for an unusually long stretch of the season. The Arab army, encamped in the open with supply lines already strained by the naval setback and Bulgarian raids, began to suffer catastrophic losses — not primarily from Byzantine arms, but from cold, starvation, and disease. Later Arab sources speak of soldiers reduced to eating their own pack animals, and some accounts, likely exaggerated for dramatic effect but nonetheless testifying to the scale of the disaster, even describe cannibalism in the camp. Whatever the literal truth, every source agrees on the essential point: the Umayyad army that had marched on Constantinople in the summer of 717 was, by the spring of 718, a shadow of the force that had set out, and it had barely fought a major land battle to become one.

Reinforcement, betrayal, and a second naval disaster

Damascus was not yet ready to give up. In the spring of 718, fresh fleets were dispatched from Egypt and North Africa to reinforce Maslama and renew the naval effort — a clear sign that the Caliphate, even after a catastrophic winter, still believed the city could be taken. But this reinforcement produced one of the most consequential twists of the entire siege. Many of the crews manning the Egyptian ships were Christian Copts, conscripted into Umayyad service rather than willing participants in a war against a Christian capital. In significant numbers, these crews defected to the Byzantines, bringing with them not just their ships but detailed knowledge of the fleet’s movements and dispositions.

Armed with this intelligence, the Byzantine navy struck again, inflicting another severe defeat on the Arab fleet in the waters around the city. Coupled with the losses already suffered from disease, starvation, and the earlier Greek Fire attacks, the Umayyad naval power that had once threatened to strangle Constantinople was, by mid-718, effectively broken.

Retreat and catastrophe

By the summer of 718, the situation had become untenable. Caliph Suleiman had died during the siege — before it had even truly begun to unravel — and his successor, Umar II, inherited a campaign that had already cost the Caliphate enormous resources for no gain whatsoever. Compounding the crisis, a Byzantine-allied force in Anatolia may have threatened Arab territory closer to home, adding further pressure to abandon the campaign. In August 718, Umar II ordered Maslama to lift the siege and withdraw.

The retreat proved almost as costly as the siege itself. As the surviving fleet sailed home, it was struck by violent storms in the Aegean and Sea of Marmara, and further losses were reportedly inflicted by an eruption of Thera (modern Santorini), whose ash and disturbed seas added to the chaos facing the retreating ships. What remained of the land army, meanwhile, had to march back through Anatolia harassed at every stage. Of the enormous force that had set out from Damascus with such confidence little more than a year earlier, only a small fraction is said to have returned home. Medieval sources claim the fleet alone returned with as few as five ships out of the many hundreds that had set out — a figure modern historians treat cautiously, but which nonetheless captures just how total the disaster was understood to be by contemporaries on both sides.

Aftermath

For Byzantium, the immediate relief was obvious: the empire’s capital, and with it the empire itself, had survived the single greatest external threat of its history to that point. Leo III, whose reign had begun in near-desperate circumstances only months before the siege, emerged from it as one of the most capable and legitimate emperors Byzantium had seen in decades, free to turn his attention to the internal reforms — including the controversial religious policy of Iconoclasm — that would define the rest of his rule.

For the Umayyad Caliphate, the failure marked a genuine turning point. The dream of taking Constantinople and pushing directly into southeastern Europe through the Bosporus was never seriously attempted again on this scale. Umayyad expansion did continue elsewhere in the years that followed — most famously into the Iberian Peninsula and across the Pyrenees into Frankish territory, a push that would be checked at the Battle of Tours in 732 — but the eastern route into Europe, through Constantinople itself, had been decisively closed.

Why it matters: The impact of the siege on world history

It is difficult to overstate how differently the history of Europe and the Mediterranean world might have unfolded had Constantinople fallen in 718. Had the city been taken, the Byzantine Empire — the last direct continuation of the Roman state, and at the time still one of the wealthiest and most sophisticated civilizations in the world — would very likely have collapsed entirely, more than seven centuries before its actual fall in 1453. The Balkans and southeastern Europe would have lain open to Umayyad expansion at a moment when Western Europe itself was fragmented, militarily weaker, and in no position to mount an effective defense of the kind that would, only fourteen years later, stop a separate Umayyad advance at Tours.

Historians have long placed the siege of 717–718 alongside the Battle of Tours as one of the two events most commonly credited with halting the westward advance of the early Islamic caliphates into Europe — one blocking the route through the Balkans, the other through Iberia and Gaul. Unlike Tours, however, the siege of Constantinople was not a single battle but a thirteen-month trial of endurance, technology, diplomacy, and sheer survival, in which no single dramatic clash decided the outcome. Instead, it was decided by the layered advantages Byzantium had spent centuries building: walls that no army had yet breached, a weapon whose secret no rival power could replicate, and a web of diplomatic relationships — with Bulgaria above all — that could be called upon in the empire’s darkest hour.

The siege’s failure also had consequences far beyond the immediate military balance. It preserved Constantinople as the guardian of Greco-Roman learning, Christian theology, and Roman law at a moment when much of that inheritance had already been lost in the West. Manuscripts, institutions, and traditions that would later feed into the Carolingian Renaissance, the Islamic Golden Age’s own engagement with Greek philosophy through later, more peaceful channels, and eventually the Italian Renaissance itself, all depended in part on Byzantium’s continued existence as a bridge between antiquity and the medieval world. Had Constantinople fallen in 718, that bridge would have been severed centuries before Europe was ready to do without it.

In the long view of history, the Second Arab Siege of Constantinople stands as one of the quiet hinge-points of the medieval world — less famous today than Tours or the later fall of the city in 1453, but arguably just as consequential in shaping the boundary between the Islamic and Christian worlds that would define European and Near Eastern history for the next thousand years.