Following the fall of Rome to the Visigoths, Constantinople took up the mantle of Roman Empire and again established rule through the Mediterranean under the emperor Justinian (527-565). Such a massive empire again proved unwieldy, and Justinian had to install massive bureaucracy to achieve the continuation of his empire. While maintaining order, the bureaucracy was also incredibly expensive, which ironically created unrest as the populace grew weary of heavy taxes despite the wealth of empire. Emperor Maurice (582-602) created cost-saving measures whenever possible, such as refusing in 598 to pay ransom to the Avar Khaganate for thousands of Byzantine prisoners-of-war. The result was the soldiers being slaughtered, but the coffers of the Empire remaining full. In 602 as another measure, he ordered the army to make winter quarters on the frontier north of the Danube rather than march home. This action caused the army to rebel and march on Constantinople, dragging Maurice out of sanctuary in a monastery to execute him. Their leader Phocas was installed the new emperor.
Although popular, Phocas proved unable to defend the empire. In the north, the Avars and their Slavic allies overwhelmed the Balkan territories. In the east, Governor Narses of Mesopotamia incited a rebellion against Phocas' rule. When Phocas sent an army to put him down, Narses sought aid from Khosrau II, emperor of the Sassanid Persians, who was pleased to attack the weakened Byzantines. The Persians defeated the Byzantine army sent against them and began conquering through Armenia and Asia Minor. In 610, Heraclius, the Exarch (“regional governor”) of Africa, overthrew the now very unpopular Phocas and tried to make peace. The Persians denied him and continued conquering the Levant and Egypt. Heraclius assembled expeditionary forces to counterattack in northern Asia Minor and then left Constantinople in 624 to campaign in the Caucasus.
The Avars continued their sweep across the Balkans to the capital itself with some eighty thousand men and siege equipment with the goal of wiping out the Byzantines altogether. An army twelve thousand strong and featuring cavalry defended the city, but it was the bureaucracy who managed life there. A bureaucrat named John determined that food the coming siege was of crucial value and began work to maintain the bread supply. He moved to cancel the free bread ration for the imperial guard (who had ample money of their own to spend) and enacted that overall bread prices be increased from three to eight folles to ensure none was wasted. On May 14 and 15, people gathered at the Great Church and chanted in protest. The local governing body under Bonos discussed what to do and ultimately decided that austerity must be retained in the face of the oncoming barbarians. After days of protest, the government sent loyal soldiers to chase away the chanters. Rioting began, and soon the city was set aflame. Order was restored at times, but the populace proved unresponsive even to zealous religious appeals. In the end, most of the citizenry abandoned the city and fled by sea in convoys to avoid attack Persians. City bureaucrats attempted to stop the retreat with control of the sea walls, but defenses were sabotaged by the people hoping to escape.
When the Avars arrived on June 29, few soldiers were left loyal to Byzantium. A short battle followed, and, despite superior defensive technology with its walls, the Avars broke into Constantinople. Barbarians looted what remained of the city and burned the rest, ending what had been a key position of trade in the known world. Heraclius found himself without a capital, and his allies lost all confidence. He began an overall evacuation to Africa and established himself there, though the empire continued to crumble with Visigoths seizing lands to the west in Spain. The Persians and the Avars reached agreement on a border along the Hellespont, giving both access to trade there while making it a dangerous haven for pirates on the newly unprotected strait.
Although victorious over their Byzantine rival, the Sassanids soon found themselves overwhelmed by the Arab Empire that grew up following the spread of Islam in the 630s and 640s. It eclipsed Zoroastrianism and spread through Africa to Spain, India, and northward to become the principal religion of the Huns and Rus. Charlemagne maintained Christendom in central Europe, and the Scandinavian nations joined as well. Western Europe continued as a marginal corner of the world with trade centering on the vast holdings of the Caliphates. Eventually European explorers seeking a westward route around the Muslim monopoly discovered the New World, which brought a new age of empire upon the out-of-the-way continent.
In reality, the government removed John (who earned the name “Seismos” or “Earthquake”) and instead worked to reinforce the spirit of the Constantinopolitans to stand against the heathen hordes. After a short siege that summer, the Avars “lacked the technology and the patience to take the city” (Walter Kaegi) and gave up when they deemed Heraclius' victories divinely inspired. Heraclius defeated the Persians and established Byzantine security, which was reaffirmed by the Crusades against the Turks beginning in 1095. Constantinople would not be conquered until 1453.