…the primary achievement of the new medieval Byzantium was to prevent Muslim efforts to capture Constantinople, which would have opened the way to a rapid conquest of the Balkans, central Europe, and probably Rome itself.
Byzantium: The Surprising Life of a Medieval Empire, by Judith Herrin
I have mentioned often the turning point represented by the victory of Charles Martel at the Battle of Tours in 732; just as often I have mentioned King John III Sobieski at the Battle of Vienna in 1683. In both cases, the turning back of Muslim forces intent on penetrating further into Europe.
This will be a story of the role of Byzantium. Despite losing significant territory to the Muslims, Constantinople was held until the mid-fifteenth century, giving Western Europe time to reform after the fall of Rome.
A decade of warfare against the Persians in the 620s and constant invasion by the Slavs into the Balkans was followed by attacks and invasion by desert tribes from Arabia into the eastern Mediterranean. At one point the situation became so dire that the emperor would relocate to Sicily in the 660s.
The defenses of the empire were stretched to the breaking point. To the west and north, Slavonic and Avar tribes would cross the Danube and capture major cities, allowing them to move south with their families to take advantage of better pastures.
Following further losses, Roman troops refused to campaign north of the Danube, instead turning to Constantinople and overthrowing the emperor. In the meantime, Persians would overrun the eastern frontier, devastating major cities in Asia Minor. Antioch succumbed, and Jerusalem was sacked. Alexandria was occupied, and the grain shipments relied on from Egypt were prevented.
In 628, however, a major victory over Persia. The Shah of Shahs was overthrown, and his palace was sacked. The True Cross, lost in the sacking of Jerusalem, was recovered. A few short years later, Mohammed died, and Arab Muslims replaced Persian Zoroastrians.
In their post-victory confidence, imperial officials refused the tribute traditionally paid to tribes who guarded the edge of the desert and had previously provided an early-warning system.
In hindsight, a mistake. By the time of his passing, Mohammed had united the disparate Arab tribes. With the death of the Prophet, the Arabs were determined to spread Islamic domination throughout the known world.
Damascus, Gaza, Antioch, Jerusalem. One by one these would fall, from 634 to 638. No one envisioned such losses were permanent, but these proved irreversible. With their capital at Damascus now established, they would plan regular campaigns into Byzantium.
Within a decade, Syria, Palestine, and Egypt would all be occupied. About two-thirds of imperial territory would be lost – and the rest was in sight. The loss of Jerusalem was a deep humiliation to the Christian world; the loss of Egypt required a complete change to the economic system.
Using their knowledge of astronomy for travel through the desert, the Arabs would turn that knowledge into travel by sea, now threatening the islands and coastlines of the empire. North Africa and southern Europe were in sight – a complete restoration of the Roman Empire, this time under Muslim Arab control.