The Eastern Roman Empire, named so since its capital, Constantinople, sat on the ancient Greek city of Byzantium, had survived for a thousand years after the fall of Rome to the Goths in the 5th century CE. Beset on all sides, the Byzantine Empire’s resilience was rooted in its flexible and efficient multi-layered defense system. The system began with a superior intelligence and diplomatic organization managed from the “Office of Barbarians”. Should an invader actually attack, they first met the buffer states, Georgia, Armenia etc which provided time for a series of well stocked and provisioned border fortresses to be manned. These strongpoints fixed invaders so they could be defeated by the free peasants of the “themes” or provinces, and the semi-autonomous regional professional armies or tagmata. All of which if necessary could be reinforced by the Emperor’s personal guard and the nobles levy from Constantinople.

This system ended with the catastrophic defeat of the cream of the Empire’s troops at the Battle of Manzikert in 1071. The Byzantine system was amazingly effective on the defense but cumbersome on the offense. In an attempt to expand and recover land lost to the spread of Islam, the emperors during the prosperous 11th century undermined their own defense by making the system so efficient it was no longer effective . Moreover they imposed crushing taxes on the thematic troops, and tried directly controlling the buffer states, namely Armenia, the bulwark of the eastern approaches. In the confusion of the Armenian War, the Seljuk Turks broke into Anatolia and crushed the Byzantine Army sent to expel them. The Byzantines would never recover. No longer would Asia Minor be solely Byzantine: a patchwork of Turkic tribes occupied central and eastern Anatolia.

Over the next 400 years, one tribe would reign supreme and unite the others – the Ottomans. Under a historically uncommon string of energetic, confident and piousleaders, the Ottomans developed their own effective system of offensive jihad. They expanded over Asia Minor and into the Balkans, leaving the Byzantine Empire with just the Peloponnese, Thrace, and the capital, Constantinople.

Constantinople was the Byzantine trump card. If all else failed, the walls of Constantinople had held. They had never been forcibly breached (They had been penetrated by the knights of the Fourth Crusade, but only because a guard left a sally port open). The first ring of the city’s defensive walls was built by Emperor Constantine when he moved the capital of the Roman Empire to the city. The second and third rings were built in the 5th century by Emperor Theodosius II. Over a thousand years, the Theodosian Walls had withstood twelve separate major sieges by the Rus, Arabs, Sassanids, Avars, Bulgars, Byzantine usurpers, and even the Ottomans.

The Theodosian Walls protected the landside approaches to the city and were 6.5 km long from the Golden Horn to the Sea of Marmara. To breach the walls attackers first had to cross a 20m wide and 7m deep moat that could be flooded on command by a series of dams controlled inside the city. The first wall oversaw the moat and the second wall had firing platforms to cover both the moat and first wall. Behind that was the massive third wall which covered the first and second walls. The third wall was 5m thick and 12m high, with 96 towers, one every 70m, providing interlocking fields of fire. The walls and food stores were maintained by the cities’ various factions in an ingenious imperial competition that saw complete obliteration of the faction if the quotas and required work weren’t met. The seaward side of Constantinople was defended by the Imperial fleet which had a secure anchorage behind a massive chain that blocked the Golden Horn. Any assault from the sea was met by the fleet which was equipped with the infamous “Greek Fire”, a flammable concoction that produced a fire that couldn’t be put out with water, and only burned hotter the more you tried to smother it. Modern chemists have not been able to reproduce Greek fire.

However, unlike prior assailants, the Ottoman host in 1453 had several previously unknown advantages. First, Ottoman possessions in both the Balkans and Anatolia isolated Constantinople from assistance by land. The final Crusade called by the Pope ended in disaster in 1444 when Polish, Hungarian and Wallachian crusaders were defeated by Ottoman Sultan Murad II at the Battle of Varna in eastern Bulgaria. The only way to relive a besieged Constantinople was by sea, and by 1452, two massive fortresses closed the Bosporus to Christian ships. Moreover, despite Pope Nicholas V’s pleas, Christendom was not prepared to send assistance: France and England were war weary from the Hundred Years War, which would finally end that autumn. The Germans were busy fighting among themselves. The Eastern Europeans were still trying to hold back the Muslim tide in the Balkans in the wake of Varna. And Spain was in the final stages of the Reconquista. Only the Italian city states could send aid, and those that ran the blockade were woefully inadequate. Finally, Sultan Mehmed II had something that no previous besieger possessed: cannon.

In 1452, a German iron founder and engineer from Transylvania (then part of Hungary) named Orban was showered with funds by Mehmed to build the new German bombards that were revolutionizing siege warfare across Europe. Orban’s largest bombard was nearly 9m long and could hurl a 275kg cannon ball nearly a kilometer and a half (Almost a mile). It was crewed by 400 men and had to be dragged by 60 oxen. Orban’s great bombard was just one of 70 cannon at Mehmed’s disposal for the siege.

On Easter Sunday, 1453, Sultan Mehmed II arrived outside the walls of Constantinople with nearly 100,000 troops, 10,000 of whom were elite Janisaaries, 70 cannon, and 125 ships. Emperor Constantine XI and his commander Giovanni Giustiniani from Genoa had just 11,000 men of which 2000 were Venetian and Genoese and 600 renegade Turks, and 26 ships safely locked behind the great chain in Golden Horn. It wasn’t nearly enough.

Mehemd II immediately, but arrogantly, launched a series of frontal assaults with predictable results. The Byzantine defenders stood firm along the Theodosian Walls just as they had for a thousand years. Constantine XI tried to buy off Mehmed II, but the Sultan wanted the city for his new capital and he knew there would be no better chance to seize it than at that moment. The sultan unleashed Orban’s bombards which over the next six weeks systematically reduced the Theodosian Walls to rubble. To further spread out the Byzantine troops, Mehmed ordered his fleet painstakingly dragged overland and launched into the Golden Horn, bypassing the great chain. On 22 April, the Byzantines attempted to destroy the Ottoman fleet with fire ships, but a deserter warned of the impending assault and the Venetian ships were sunk before they could do damage. The surviving Venetian sailors were impaled on the north shore. In response, Constantine XI ordered the execution of all Ottoman captives, one at a time and in full view of the Ottoman army. The Ottoman fleet built massive floating firing platforms in the Golden Horn which forced the Byzantines to man the sea walls, spreading their few troops dangerously thin.

At night the Byzantines repaired the damage to the walls as best as they could and during the day they countermined. As the Ottomans pounded the walls from above, German and Serbians mercenary sappers undermined the walls from below. Throughout May 1453, dozens of small vicious battles occurred below ground as mines and countermines intersected. In the flickering torchlight, groups of nearly naked men fought with picks, shovels, knives, and fists against foes identified only by the language they screamed in the darkness. After capturing two Turkish officers, the Byzantines knew the locations of all the mines and successfully shut down the Ottoman mining operations. But it was just delaying the inevitable: the Theodosian Walls were breached in more than few places, and Constantine and Giustiniani simply didn’t have enough men to plug the gaps effectively. And no relief force was enroute. The Byzantines were doomed.

On 28 May, as the Ottomans were openly preparing for their final assault, the Byzantines and Italians held religious parades culminating with a co-denominational mass in the Hagia Sophia with both the Italian and Byzantine nobility in attendance. That mass was the first time Roman Catholics and Greek Orthodox Christians celebrated mass together since the Great Schism of the 11th century and was the last Christian mass in the Hagia Sophia to this day.

On the morning of Tuesday, 29 May, 1453, as the moon waned in the sky, three great Turkish waves crashed against the Theodosian Walls and the sea walls along the Golden Horn. The first two were comprised of irregulars, Serbians, and Anatolian troops and were driven off with great loss by the Byzantines. They did however serve their purpose, they sufficiently weakened and disorganized the defense which was promptly exploited by the Janissaries. In short order, Giustiniani was mortally wounded, and his evacuation from the walls caused the Italians to collapse. Doffing his imperial regalia, Constantine was last seen leading a final futile charge against the Janissaries occupying the Kerkaporta gate. His body was never recovered. The remaining Byzantine soldiers fled home to protect their families while the Venetians and remaining Genoese fled to the harbor to escape.

That evening, Mehmed II rewarded his army with three days of loot, arson, murder, and rape in the city. Battles among the Turks erupted over the slaves and spoils. At the end of the three days, 20,000 of the 50,000 inhabitants of Constantinople were massacred, and the rest were sold into slavery. The Hagia Sophia was turned into a mosque. The Byzantine Empire was destroyed and the Greek world would never recover. Ancient Rome’s legacy would live on for another thirty years in the Byzantine rump states of Trebizond and Morea. And with rare exceptions, the Ottoman Empire would go on to nearly unchecked expansion for another 220 years.

In the Greek world, Tuesday is known as a day of bad luck. And Turkey is the only Islamic state whose national flag features not a crescent, but a waning moon.