In the mid-6th C., the nomadic tribe of the Avars crossed the plains to the north of the Black Sea and reached from Asia to the Danube region, at the Byzantine frontier. Their arrival would overturn the balance of power at the northern frontier of the Empire and played a huge role in the Slavic settlement at the Balkan Peninsula. Already in 558, the Avars sent an embassy to Emperor Justinian I, offering their aid against some nomadic tribes which were threatening the Empire those days. In 567 the Avars settled in Pannonia, where they formed an alliance with the Lombards and destroyed the state of the Gepids. The following year, the Lombards moved towards North Italy, leaving the domination of the valley of Pannonia to the Asian settlers. The Avar tribes formed a mighty alliance, into which the remnants of the defeated Gepids, part of the Slavs and the Bulgarians were incorporated. Without further ado, they turned on the Byzantine towns along the frontier: in 582 they took Sirmium, and in 584 Singidunum, Viminacium and Augusta. Soon afterwards they descended southwards and in 584 and 586 besieged Thessalonica without success. Their offensive against the Empire was continued during the reign of Maurice and reached its climax during the reign of Herakleios (610-641). Because the emperor was occupied with the war against Persia that lasted many years, he was forced to form a peace treaty with the Avars, which entailed a high annual tribute. During the meeting with the Avar khagan in 617, he almost fell a victim to an ambush It was becoming clear that the aim of the Avars was taking over the European territories of the Byzantine Empire, including capital city of Constantinople.
2. The siege of Constantinople by the Avars
2.1. The blocade and the first operations (June 29 - July 30, 626)
The siege of Constantinople in 626 was a key event in the relations between Byzantium and the Avars. The latters were scheming for a long time to capture the Byzantine capital and they were not even abashed by their unsuccessful attempt against the emperor in 617. In spite of the large grants they received in 619 by the Byzantines – an annual tribute of 200.000 golden coins and prominent hostages – the ambitious khagan did not suspend his preparations for the siege of Constantinople. Taking advantage of the difficult situation for Byzantium in the eastern frontier, where Herakleios was from 622, the Avars violated the peace treaty, and along with the Persians they decided to lay siege on the Byzantine capital. Early in June 626, the Persian general Shahrbarāz appeared with his army before Chalcedon, on the coast of Bosporos in Asia Minor. While waiting the arrival of the khagan, Shahrbarāz set on fire the suburbs of Chalcedon, churches and mansions.
On Sunday the 29th of June 626, the advance guard of the Avars, counting almost 30.000 men, reached from Adrianople in front of the Long Wall, a defense system which the emperor Anastasios I had erected in the late 5th century. This was a high wall at a distance of 65 kilometers from Constantinople, that run from the coast of the Sea of Marmara to that of the Black Sea. The same day the Byzantine army retreated from the suburbs inside the walls of Constantinople. Inside the city great disturbance prevailed, and some signs of increased panic appeared. The Byzantines sent envoys to the khagan stating that they were ready to satisfy his will if he lifted the blocade. In the meantime magistros Bonus, regent at the capital along with in the patriarch of Constantinople Sergios, hastened to organise the defense of the city. Sergios on the other hand was trying to calm down and to encourage the worried citizens. Emperor Herakleios, informed in time about the intentions of the Avars, sent a battalion from the far away region of Lazika, where he had encamped with his army, and gave orders for the organisation of the defense.
Soon after that, around a thousand soldiers of the khagan reached the eastern district of Sykae and exchanged signal fires with the Persian forces at Chrysopolis, on the Asian coast of the Bosporos. In the area between Chalcedon and Chrysopolis the burnings did not cease at all, while signal fires appeared over the western suburbs of the city as well. Then the Avars destroyed the aqueduct that supplied Constantinople with water. Inside the capital, the anxiety and the fear were growing bigger.
Sure of his imminent and definitive success, the leader of the Avars, whose name is not preserved in the surviving sources, received at Adrianople a new embassy, sent by magistros Bonus and the Byzantine dignitaries. The Byzantines were confident in their strengths and they refused to surrender or leave the city. angry the khagan, whom the Byzantine writers call pig, leech, beast, wily snake as well as with the most insulting names, refused to accept the Byzantine envoy, showing indisputably that there would be no negotiations any more. An infuriated khagan declined the envoy and made clear that there would be no more negotiations.
In the small hours of the morning of Tuesday, July 29, 626, the main body of the Avar army closed in before the walls of the Byzantine capital. According to the sources of the period, it counted around 80,000 men, including the forces of the subjucated Gepids, Slavs and Bulgars. Anticipating the clash, magistros Bonus inspected the troops along the walls and issued the last orders. Patriarch Sergios performed a massive litany to hearten the defenders of the city. The next day the Avars brought forth their siege machines and made the last preparations for their assault.
2.2. The attempts against the city walls (July 31 - August 6, 626)
At the dawn of July 31 the Avars launched their attack, which is described by the contemporary poet and eyewitness Theodore Synkellos: he says that the Avars attacked to the city walls “rumble, thunder and hail.”1 On the front line fought the lightly armed Slavs, and on the second one the heavy armored infantry of the Avars. Already in the first day the besiegers suffered heavy losses. This lifted the spirits of the defenders, who believed that they had on their side the Theotokos, whom they considered as the patron of the city.2 On the next day, August 1, the Avars kept up their offensive with their siege machines: a dozen of wooden towers covered with skins and almost as tall as the walls. However, the defenders managed to burn some of the wooden towers, prompting the Avars to retreat.
The khagan also organised an attack by sea, knowing that Constantinople could not be conquered without navy forces and that th Golden Horn was the weakest point of the city's defences. But since the Avars were a people of the land, with no naval experience wathsoever, the kgagan had ordered that this operation should be undertaken by the Slav monoxyla, little vessels cut in the trunk of trees.
In the meantime, the Byzantines were making further diplomatic efforts. Magistros Bonus proposed for yet another time to the Avars to lift the siege in exchange for tribute and generous presents. But the khagan once more made excessive and unacceptable demands, particularly when demanded from the population to abandon the city and leave their fortunes behind.
On Saturday, 2 August, the fights continued; however, the situation remained more or less unchanged. The khagan gravely warned the Byzantine embassy who had asked to come to his camp, that if the city did not surrender, he would destroy whatever found on his way. He threatened to transport three thousand Persians to the European coast of Bosporos and he even presented the Persian envoys of Shahrbarāz. The Byzantines did not accept this ultimatum and dropped the negotiations. On the same night they managed to capture the Persian envoys who were on their way back to Chalkedon. One of them was executed on the spot and the other two were dragged into the city. In the next morning, they were carried on the walls and left exposed to the besiegers' view. The Byzantines cut off the hands of one of them and then sent him back to the Avars, along with the head of the envoy whom they had killed the previous night. Then they dragged the second one to Chalcedon, where they beheaded him before the Persians and threw his head on the seashore.
In the dawn of August the 4th, a sea battle took place between the Byzantine fleet and the Slav monoxyla. In this uneven battle the flotilla of the monoxyla was crashed and their crew was killed or got drowned. On the seventh and the eight day of the siege (4-5 August) the khagan was hastily preparing for the final and decisive assault, and only skirmishing and local attacks took place. On Wednesday, August the 6th, a great fight broke out all along the walls, and continued into the night. The Avars suffered heavy losses, while on the Byzantine side the losses were much less.
2.3. The last assault and the retreat of the Avars (August 7, 626)
The final assault by land and sea was launched on the morning of August the 7th. The screams and the voices of the Avar besiegers broke the skies. The Slav monoxyla appeared in the Golden Horn. The khagan counted on his land army to break into the city wall, and on the Slav boats to secure a way into the city by way of the Golden Horn and the region of Blachernae, where the fortifications were most weak. But, although the Avars managed to occupy the church of the Theotokos of Blachernai and the surrounding area and to barricade themselves there, they suffered so many losses that they were not even able to gather their injured and their dead men. At the sea battle, the Byzantine fleet surrounded and destroyed the Slav monoxyla, and the Patriarch Nikephoros mentions that the sea became red from the blood.3 Theodore Syncellos writes that the sea near Blachernai was covered with dead bodies and empty boats floating aimlessly. Only a small number of the Slavs managed to get rescued. The khagan, who watched the course of the battle from a nearby hill, descended on foot, went to his tent and began hitting his chest and head with his feasts in anger and despair. Scared that there might be retaliations on the part of the Avars, the Slavs abandoned their places and escaped in the mountains. The cavalry of the Avars turned to pursue them and suddenly almost the entire area in front of the walls of Constantinople remained deserted.
An exhilarated group of the city's defenders, among which women and children, attempted a sortie. But the ever cautious magistros Bonus ordered them back inside immediately. In the night, and under the protection of the darkness, the Avar warriors came back to gather the remnants of their siege machines and they burnt all of them along with their encampment. It was a symbolic recognition of their defeat. On the morning of August the 8th, 626, in front of the walls of the capital there was not even one Avar soldier.
Despite the sense of relief and the celebrations for the victory at Constantinople, people would not come out of the city because some Avar troops were roaming the suburbs burning down whatever was on their way. The Byzantines rejected the proposal of the Avars for new negotiations. Realising that the aim was anattainable, the khagan abandoned the region of Constantinople. The Persian army, who did not participate in the battle but had waited, remained in the region of Chalcedon until the spring of 627, but this was of no strategic importance anymore.
The Byzantines retained the memory of the city's salvation by means of an annual celebration on August the 7th.
As for the factors of the Avars’ defeat before the walls of Constantinople, the lack of provisions, the erroneous tactics at the sea, and the ethnic heterogeneity of the invaders (Avars, Slavs, Bulgarians, Gepids) should all be mentioned, along with the fact that it had been too ambitious an operation in the first place. The Avars never again fully regained their power after this failure, and their hold on the various tribes from a wide area - from the Central Europe to Caucasus - was diminished considerably. The empire of the Avars continued to exist until the beginnings of the 9th century, when it was crushed by Charlemagne, but it never regained its former strength and prestige. For the Avars, the defeat in front of the gates of Constantinople was the beginning of the end.