In the seventh century, the eastern Roman Empire faced threats so severe that it came close to destruction. It lost most of its territory but survived, transformed into a smaller, tougher, Greek-speaking state. Historians often see this as the moment the eastern Roman Empire became recognizably “Byzantine.”

The last great war of antiquity

For centuries, Rome's great rival in the east had been the Sasanian Persian Empire. In 602 a military rebellion overthrew the emperor Maurice, and the Persian king Khosrow II used the chaos to invade. Persian armies swept through Syria and Palestine, taking Jerusalem in 614 and carrying off the relic believed to be the True Cross. They conquered Egypt, the empire's breadbasket. In 626, Persian forces and their allies, the nomadic Avars, besieged Constantinople itself. The walls held.

Heraclius

The emperor Heraclius, who ruled from 610 to 641, turned the war around. Instead of defending, he led armies deep into Persian territory. In 627 he won a decisive battle near the ruins of Nineveh, and Persia, torn by its own civil strife, made peace. Heraclius recovered the lost provinces and returned the True Cross to Jerusalem in triumph. It seemed a great victory, but both empires were exhausted.

The Arab conquests

Almost immediately, a new power emerged from Arabia. United by the new religion of Islam, Arab armies began to expand after the death of the Prophet Muhammad in 632. At the Battle of the Yarmuk in 636, they crushed a Roman army. Syria, Palestine and Jerusalem fell. Egypt was conquered by about 641, and Alexandria followed. Arab forces also destroyed the Sasanian Persian Empire entirely. Within decades, the Roman Empire had lost most of its richest provinces, and North Africa followed by the end of the century.

Many local Christians did not resist strongly. Some belonged to churches that Constantinople had persecuted as heretical, and early Arab rulers generally allowed Christians and Jews to keep their religion if they paid a special tax.

Greek fire

The Arabs now aimed at Constantinople itself. In the 670s Arab fleets attacked the city repeatedly. The Romans had a secret weapon. Around 672, according to later chroniclers, a refugee architect from Syria named Kallinikos brought a new invention: a liquid incendiary that could be sprayed through tubes mounted on ships, bursting into flame and burning even on water. Westerners later called it Greek fire. It devastated enemy fleets. Its recipe was a closely guarded state secret, and it was eventually lost; historians still debate its ingredients, though crude petroleum was probably a key component.

The great siege of 717

In 717 to 718, a huge Arab army and fleet besieged Constantinople by land and sea for about a year. The emperor Leo III, only recently come to power, defended the city. Greek fire destroyed many Arab ships, a harsh winter and disease weakened the besiegers, and Bulgar allies attacked them from the rear. The siege failed. Some historians compare its importance to the Frankish victory at Tours in 732: had Constantinople fallen, the history of Europe might have been very different.

A transformed state

The empire that survived was very different from Justinian's. It was smaller, centered on Anatolia and the Balkans. Greek replaced Latin as the language of government; Heraclius adopted the Greek title basileus for emperor. Cities shrank, and much of life moved to fortified towns. Provinces were reorganized into military districts called themes, in which soldiers were settled on land and defended their own regions. The empire adapted to a long struggle on many frontiers.

Cutting off noses

Byzantine politics could be brutal. There was no fixed law of succession; an emperor needed the acclaim of the army, the people and the senate, and could be overthrown. A common way to remove a rival without killing him was mutilation, since a ruler was expected to be physically whole. In 695 the emperor Justinian II was overthrown and his nose was cut off. Remarkably, he returned to power in 705 with the help of the Bulgars, wearing, by some accounts, a golden nose. Blinding was also used to disqualify rivals.

Worked example: how did the empire survive?

Many states would have collapsed after losing two thirds of their territory. Weigh the reasons Constantinople survived.

• Geography and walls. The city was surrounded by water and protected by the Theodosian Walls.
• Technology. Greek fire gave the navy a decisive advantage.
• Adaptation. The theme system created local defense forces with a stake in their land.
• Leadership. Emperors such as Heraclius and Leo III rose to crises.
• Diplomacy. The empire skillfully used alliances, payments and divisions among its enemies.

The lesson is that resilience comes from many sources at once. A state that can defend its core, adapt its institutions and use diplomacy as well as force can survive disasters that would destroy a more rigid one.