Siege of Constantinople (626) in the context of "Byzantine–Sasanian War of 602–628"

The walls of Constantinople (Turkish: Konstantinopolis Surları; Greek: Τείχη της Κωνσταντινούπολης) are a series of defensive stone walls that have surrounded and protected the city of Constantinople (modern Fatih district of Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built.

Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger. They saved the city, and the Byzantine Empire with it, during sieges by the Avar–Sassanian coalition, Arabs, Rus', and Bulgars, among others. The fortifications retained their usefulness even after the advent of gunpowder siege cannons, which played a part in the city's fall to Ottoman forces in 1453 but were not able to breach its walls.

Early Slavs began mass migrating to Southeastern Europe between the first half of the 6th and 7th century in the Early Middle Ages. The rapid demographic spread of the Slavs was followed by a population exchange, mixing and language shift to and from Slavic.

The settlement was facilitated by the substantial decrease of the Southeastern European population during the Plague of Justinian. Another reason was the Late Antique Little Ice Age from 536 to around 660 CE and the series of wars between the Sasanian Empire and the steppe nomads against the Eastern Roman Empire. After the arrival of the Pannonian Avars in the mid-6th century, they continued to conduct incursions into Roman territory, often independently of Avar's influence. After the failed siege of Constantinople in the summer of 626, and successful revolt against the Avars, they remained in the wider Southeast Europe area after they had settled the Byzantine provinces south of the Sava and Danube rivers, from the Adriatic Sea to the Aegean and Black Sea.

Sergius I of Constantinople (Greek: Σέργιος, Sergios; died 9 December 638) was the Ecumenical Patriarch of Constantinople from 610 to 638. He is most famous for promoting Monothelitism Christianity, especially through the Ecthesis.

Sergius I was born of Syrian Jacobite heritage. He first came to power as Ecumenical Patriarch of Constantinople in 610. He was also a known supporter of Emperor Heraclius, crowning Heraclius as emperor himself in 610. Sergius I also provided support to Heraclius throughout his campaign against the Persians. Sergius I also played a prominent role in the defense of Constantinople against the combined Avar-Persian-Slavic forces during their invasion of Constantinople in 626. Sergius I's connections to both political and religious authorities gave him to his influence in both the religious and political communities to further Monoenergism as the primary formula of Christ within the church. This was met with much opposition, especially from that of the Chalcedonian supporters, Maximus the Confessor and Sophronius of Jerusalem. In response to their resistance to accept the ideas of Monoenergism, Sergius I responded with the Ecthesis, a formula which forbade discussing the idea that the Person of Christ had one or two energies in favour of Monothelitism being the idea that the Person of Christ had two natures that were united by a single will. The Ecthesis was signed by Heraclius in 638, the same year that Sergius I died.