Time was pressing. It was not until March 717 that the new emperor entered his capital, Constantinople, through the Golden Gate. But there was little time to celebrate. Because an Arabian fleet of allegedly 1800 ships fought its way along the coast of Asia Minor. Caliph Sulaiman wanted to use it to finally conquer Eastern Rome after his brother and predecessor al-Walid I had succeeded in destroying the Visigoth kingdom in Spain.

The new Emperor Leon III. (ca. 680-741), often referred to as “Isaurians” because of an incorrect tradition, had at least two trump cards in his hand. On the one hand, the supply depots in Constantinople had been filled and its walls repaired, on the other hand, he had proven himself militarily as a “strategist” on the subject of Anatolicon.

This “theme” constitution had secured Byzantium’s world-historical survival in the turbulent decades before. These were military districts led by a general (Strategos) and in which stratiotes were settled, peasant soldiers who managed their farms in peacetime but could be quickly mobilized in the event of a conflict.

From his administrative district in Asia Minor, Leon had successfully opposed Theodosius III. raised, the sixth emperor to rise to the throne through rebellion in the past 20 years. It was probably the ongoing internal political crisis that Sulaiman intended to exploit for his campaign. However, Theodosius had already recognized the danger and ordered the installation of sufficient supplies. However, since the residents and troops of the capital left no doubt that they would rather leave their defense in the hands of Leon, Theodosius gave up and retired to a monastery.

Leon fulfilled the expectations placed in him. When the caliph’s brother, Maslama, appeared before Constantinople with 80,000 men on August 15, 717, the walls and equipment were in far better condition than in 678, when an Arab army had first attempted a siege of the city. Back then, the invention of a Greek architect and alchemist from Helliopolis, Syria, who snuck through the containment rings ensured the survival of Byzantium: Greek fire.

Since this “pyr thalassion” (wildfire) or “pyr hygron” (liquid fire) was traded as a state secret, its composition is not entirely clear to this day. It was probably a mixture of saltpetre and petroleum, which was sprayed onto enemy ships from bronze siphons using a pressure pump.

Since this fire could not be extinguished with water and also burned at sea, it was a fearsome weapon that required the utmost discipline to use. With the dromon, the Byzantines had developed a fast and manoeuvrable war galley that was clearly superior to the Arabian ships, which often had to be built from badly stored wood. The noise generated during combustion also acted as a psychological weapon.

Maslama began cutting off Constantinople’s land walls from the outside world with a moat. On September 1, the Caliph entered the Bosphorus with his fleet. He carried numerous supply ships with him in order to be able to maintain the siege even during the bad season, which was only possible to a very limited extent in 678.

There was an unexpected calm, which made the low-lying Arab ships largely unable to manoeuvre. Immediately, Leon launched his “lightships,” which were anchored in ports within the sea walls and in the Golden Horn, and set fire to the Arab transporters “with God’s help,” writes the chronicler Theophanes Confessor. “Some of the ships were driven to the sea wall in flames, while the others and their crews were sunk in the depths of the sea.”

This not only thwarted the planned attack on the sea walls of Constantinople, but also made it difficult to supply the besiegers over the winter. Already at the end of September an illness took away Caliph Sulaiman. His successor, Umar, decided to continue the siege, which cost “a lot of horses, camels, and other animals,” writes Theophanes.

The decision was made at sea. When two Arab fleets initially solved the supply problems in the spring, their admirals decided to take their ships to safety in a bay from Greek fire. Christian members of the crew betrayed this to the Byzantines. The “tubes filled with liquid fire on battleships and two-oarers” were used again. “Through the intercession of the Immaculate Mother of God, God helped them and so the enemies were thrown into the sea,” Theophanes rejoiced.

In addition, since a Byzantine army blocked the land routes, “the Arabs suffered from a great famine, so that they ate up all the dead animals, horses, donkeys, and camels. Some report that they also threw corpses, even their own excrement, into the oven, cured them and ate them.” When a terrible plague broke out and the Bulgarians advanced from Thrace with an army, “the new caliph allowed Masalma” to return .

For Byzantium it was a historic victory. From then on, the caliphs accepted its existence and refrained from further large-scale attacks or even from invading Europe from the east. When 15 years later the Franks under Karl Martell stopped a Muslim army at Tours and Poitiers, the danger in the West was also averted. The annihilation of the Arab fleets also meant that the eastern Mediterranean remained under Byzantium’s control.

Leon III subsequently proved to be a successful ruler who carried out numerous reforms. One of these was religious. Around 726 he ordered the removal of the large icon of Christ in front of the imperial palace, opening the epoch of iconoclasm, the great iconoclasm in the Eastern Church. It was intended to contribute to the alienation between the Western and Eastern Churches.

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