Siege of Constantinople (626) in the context of "Golden Gate (Constantinople)"

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.

The Byzantine–Sasanian War of 602–628, also called the Last Great War of Antiquity, was fought between the Byzantine Empire and the Sasanian Empire. It was the final and most devastating conflict of the Roman–Persian wars (54 BC – AD 628). The previous war between the two powers had ended in 591 after the emperor Maurice helped the Sasanian king Khosrow II regain his throne. In 602, Maurice was murdered by his political rival Phocas. Khosrow declared war, ostensibly to avenge the death of the deposed emperor Maurice. This became a decades-long conflict, the longest war in the series, and was fought throughout the Middle East, the Aegean Sea, and before the walls of Constantinople itself.

From 602 to 622, the Sassanians gradually conquered much of the Levant, parts of Anatolia, and—for the first time—Egypt and several islands in the Aegean Sea. The counter-attacks of the new Byzantine emperor Heraclius from 622 to 626 eventually forced the Persians onto the defensive. Allied with the Avars and Slavs, the Persians attempted to take Constantinople in 626, but were defeated. In 627, allied with Turks, Heraclius invaded the heartland of Persia. After the Battle of Nineveh (627), Iranian forces were finally broken, forcing civil war-torn Persia to seek peace.

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.