Byzantine Empire in Turmoil: Phocas' Reign and the Sassanid Invasion

 Background to Byzantine Victory in 626 Siege

Byzantine Victory in 626 Siege of Constantinople: The Byzantine Empire successfully defended Constantinople against the Sassanid Persians, Avars, and allied Slavs in 626. Their victory saved the empire from collapse. Emperor Heraclius’ previous and later victories allowed the empire to regain its territory and end the Roman-Persian Wars through a treaty.

During the 7th century, the Western Roman Empire was crumbling as the Goths, Franks, and Lombards built a new, medieval world. Meanwhile, the Eastern Roman Empire struggled for survival. Constant war with the Persians had left the Eastern parts of the kingdom in ruins. The Romans were unable to prevent the fall of Jerusalem in 614 and Alexandria in 619. The empire also faced invasions from the northern Avar and Slavic tribes, who plundered and vandalized their way to the gates of Constantinople in 626

In 602, Phocas seized power from Emperor Maurice, whose reign was marked by atrocities and administrative incompetence. Phocas’ mismanagement left the Byzantine Empire vulnerable and unstable, providing an opportunity for Sassanid king Khosrau II to invade. Initially, the Sassanid invasion was successful, with the Byzantines driven into Anatolia. However, Phocas was later overthrown by Heraclius, the son of the Exarch of Carthage.

Heraclius actively led his army into battle, but the Byzantine empire lost all its Near East possessions and Egypt. Despite counter-offensives into Mesopotamia, Heraclius could not stop the Persians from laying siege to Constantinople. In May 626, riots in Constantinople erupted over the cancellation of bread rations for the guards and a rise in bread prices. The leader, John Seismos, was removed, but the disturbances continued.

The Roman Empire’s political situation had rapidly declined since Justinian’s rule (527–565). Its archenemies, the Persians, had overrun the eastern regions with constant war. In the summer of 626, a Persian force stood ready to attack Constantinople across the Bosporus strait. The city also faced invasions from Avars and Slavs from both land and sea. The invaders pillaged surrounding towns and villages, cutting off supplies to the capital and setting up war equipment. Their leader, the Chagan, had nearly captured the emperor a few years earlier. The threat was more dire this time as the emperor was away on a campaign against the Persians and couldn’t intervene in time. The heavy responsibility of defending the city fell on two men, the Patriarch Sergios and the Roman general Bonos, who sought to prevent panic and quickly organized the city’s defense.


The Siege against Byzantine Constantinople

Khosrau’s Strategy

Khosrau recruited two new armies to defeat the Byzantines. He entrusted Shahin with 50,000 men to stay in Mesopotamia and Armenia to prevent Heraclius from invading Persia. A smaller army under Shahrbaraz slipped through Heraclius’ flanks and made their way to Chalcedon across the Bosphorus from Constantinople.

Khosrau also coordinated with the Khagan of the Avars to launch a coordinated attack on Constantinople from both European and Asiatic sides. The Persian army stationed themselves at Chalcedon, while the Avars placed themselves on the European side of Constantinople and probably destroyed the Aqueduct of Valens.

Heraclius’s Strategy

However, the Byzantine navy’s control of the Bosphorus strait prevented the Persians from sending troops to the European side to aid their ally, reducing the effectiveness of the siege. Additionally, the Persians and Avars had difficulties communicating across the guarded Bosphorus.

Patriarch Sergius and patrician Bonus commanded the defense of Constantinople. Heraclius divided his army into three parts, sending some reinforcements to Constantinople to boost morale, another part under his brother Theodore to deal with Shahin, and the third and smallest under his own command to raid the Persian heartland.

Assault by Persians, Avars and Slavs

The Avars were closing in on the city walls, armed with heavy siege engines, and the Persians had set up camp on the Asian side of the Bosporus, ready for attack. The citizens of Constantinople were alone, as Emperor Heraclius was away campaigning in the east. But they refused to give up. They hoped for divine assistance and worked together to defend the city walls against the Avar attack and fend off a potential Slavic invasion by sea.

On June 29, 626, the Avars and Sclaveni (Slavs) launched a coordinated assault on the walls with the intention of laying a siege with 80,000 soldiers against 12,000 well-trained Byzantine cavalry troops (presumably dismounted). Despite continuous bombardment for a month, the defenders were able to hold off the attackers with the help of Patriarch Sergius’ religious fervor and the belief that they were under divine protection. The attackers were also defeated in two naval engagements, and panicked, abandoning the siege, believing that divine intervention had won the day for Byzantium.

Byzantine ships surrounded and destroyed a fleet of Persian rafts ferrying troops across the Bosphorus on 7 August. The Avars, along with their Slav allies, attempted to attack the sea walls from across the Golden Horn while the main Avar force attacked the land walls. Patrician Bonus’ galleys successfully rammed and destroyed the Slavic boats. The Avar land assault from 6 August to the 7th also failed.

With news of Theodore’s victory over Shahin, the Avars retreated to the Balkan hinterland within two days, never again seriously threatening Constantinople. Even though Shahrbaraz’s army was still encamped at Chalcedon, the threat to Constantinople was over. In thanks for the lifting of the siege and the supposed divine protection of the Virgin Mary, an unknown author, possibly Patriarch Sergius or George of Pisidia, wrote a new proemium for the celebrated Akathist Hymn.


The Avar Siege of Constantinople in 626 by Martin Hurbanic
The Avar Siege of Constantinople in 626 by Martin Hurbanic
Map of Constantinople (modern Istanbul) and its environs during Byzantine times.
Map of Constantinople (modern Istanbul) and its environs during Byzantine times.

The Aftermath

The Byzantine army emerged victorious in the siege of Constantinople after news of another Byzantine victory led by Heraclius’s brother Theodore reached them. Heraclius then managed to turn the Persian general Shahrbaraz to his side by showing him intercepted letters from Khosrau ordering his death. Shahrbaraz then moved his army to northern Syria, giving Heraclius a strategic advantage.

The next year, Heraclius led an invasion into Mesopotamia, defeating another Persian army at Nineveh and marching on to Ctesiphon. The Persians eventually had to withdraw all armed forces and return Egypt, the Levant and any territories of Mesopotamia and Armenia that were in Roman hands at the time of an earlier peace treaty in c. 595.

The war ended and neither the Persians nor the Byzantines would fight again until the Arab-Islamic invasion.

The Avars failed to conquer Constantinople in the 626 siege due to their lack of patience and technology. Despite the Persians’ expertise in siege warfare, the walls of Constantinople easily defended against their siege towers and engines. One major factor was that the Persians couldn’t move their equipment to the heavily guarded European side of the Bosphorus, where the Avars and Slavs were stationed. Additionally, the Persians and Slavs didn’t have a powerful enough navy to bypass the sea walls and establish communication. Ultimately, the Avars abandoned the siege due to lack of supplies.


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Featured Image

Amirdovlat Amasiatsi
Byzantine Victory in 626 Siege: The Siege of Constantinople in 626 by the Avars on a mural at the Moldoviţa Monastery, Romania

The Siege of Constantinople in 626 by the Avars on a mural at the Moldoviţa Monastery, Romania

The Siege of Constantinople (626) by the Avars on a mural at the Moldoviţa Monastery, Romania. The siege depicted in actual fact is the “Fall of Constantinople to the Ottomans in 1453, as illustrated by the presence of artillery and the dress of the besieging forces. The church is one of the Painted churches of northern Moldavia listed in UNESCO’s list of World Heritage sites.

These murals of the Siege of Constantinople date the early sixteenth century, and represent key historical narratives about Constantinople’s miraculous deliverances during the 7th, 8th, and 9th centuries.

Painted on the exteriors of several churches from the region, this mural type articulated a view of history as a series of interventions, and presented a visual commentary to Moldavia’s contemporary military and spiritual struggles.



  • Wikipedia Contributors. (2022, October 15). Siege of Constantinople (626). Wikipedia; Wikimedia Foundation.

  • The Siege of Constantinople at Moldovița Monastery. (2017).

  • The Avar Siege of Constantinople in 626. (2019). Google Books.

  • Threatened orders — The Siege of Constantinople. (2023).‌

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