Between 29 July and 7 August 626 an Avar army stormed the land walls of Constantinople while a Persian army watched from the Asian shore, unable to cross. The failure broke Avar power and ended Persia's last chance to finish the war.
For ten days at the end of July and the beginning of August 626, Constantinople was attacked from Europe and Asia at once. The Avar khagan brought his own army and contingents of Slavs, Bulgars and Gepids against the land walls; a Persian army under Shahrbaraz stood on the Asian shore at Chalcedon, close enough to see the city it had come to take.
The emperor was not there. Heraclius had been campaigning in the Caucasus since 622 on the calculated judgement that the capital could defend itself while he attacked Persia where it was weakest, and he refused to come back. Command fell to the patrician Bonus, the patriarch Sergius, and his own young son Constantine.
The two enemy armies never combined, and that is the whole story of the siege. Between them ran the Bosphorus and the Golden Horn, and the Roman fleet held both. A coalition that looked overwhelming on a map was, on the water, two separate forces that could not reach each other.
By 626 the war that Khosrow II had begun in 602 had stripped the eastern empire of Syria, Palestine and Egypt. Shahrbaraz's army had reached Chalcedon, opposite the capital, as early as 615 and had never gone away.
The Avars had been a separate problem for fifty years, extracting tribute that rose from 80,000 gold solidi under Tiberius II to around 120,000 in the early seventh century. At a meeting near Herakleia — the sources date it to either 617 or 623 — the khagan attempted to seize Heraclius himself, and the emperor escaped back to the city in disguise.
What made 626 different was coordination. An Avar assault on the walls and a Persian army on the far shore were each survivable; together, with the Persians ferried across to attack the sea walls, they were not. Heraclius sent part of his army back to the city, kept the rest in the east, and did not return.
The Theodosian land walls had been built between 408 and 413 and rebuilt after the earthquake of 447. They ran roughly four miles from the Sea of Marmara to the Golden Horn as three lines: a broad moat, a low outer wall, a terrace, and behind it an inner wall some twelve metres high carrying ninety-six towers. Nothing in the sixth or seventh century could reduce that quickly.
The weak point was the northern end. The Blachernae quarter, with its church of the Theotokos, lay outside the main Theodosian line and was covered by a single wall, and it was there that the Avars pressed hardest.
The Chronicon Paschale puts the garrison at about twelve thousand — heavily outnumbered, though against these walls numbers mattered less than the second asset. The fleet controlled the Bosphorus and the Golden Horn, which meant it controlled whether the besiegers could ever become one army.
An Avar advance force appeared before the walls on 29 June and spent a month cutting the city off and burning the suburbs. The khagan arrived with the main army on 29 July and began the assault the next day.
He brought siege towers and twelve stone-throwing engines protected by hides against the stretch of wall between the Polyandrion and the Gate of Saint Romanus, and pressed the Blachernae end hardest of all. The defenders burnt the engines and held the line, but the attacks were serious and sustained rather than a demonstration.
The decisive move was on the water. Slav dugout canoes — the monoxyla — were carried overland and launched into the Golden Horn to ferry Persian troops across from Chalcedon and to attack the sea wall where it was thinnest. On 7 August the Roman ships met them in the Horn and destroyed the flotilla; the Slavs who reached the shore were killed by the Armenian troops at the Blachernae. The great land assault of the same day failed with it, and on 8 August the khagan burnt what siege equipment he had left and withdrew.
Shahrbaraz's army spent the siege on the Asian shore and never fought. This was not caution. The Persians had no fleet, and every attempt to put them across the strait depended on Avar and Slav boats that the Roman navy could sink at will.
They could not be reinforced either. Heraclius had sent his brother Theodore against the other Persian army in Armenia under Shahin, and beaten it, so there was no second force to change the balance in Anatolia while the coalition was in front of the city.
The result was the most expensive stalemate of the war. Persia had spent twenty years reaching the shore opposite Constantinople, and when it finally arrived there in company with an ally at the walls, the strait made the two armies useless to each other.
Avar power never recovered from the failure. The khaganate's authority over its subject peoples rested on success and plunder, and its subjects drew the obvious conclusion: the Slav revolt under the Frankish merchant Samo was already running in the west, and the Avar grip on the Balkans loosened permanently over the following decades.
For Persia it was the last opportunity. Shahrbaraz withdrew from Chalcedon, Heraclius was left free to campaign in the Persian heartland, and in December 627 he destroyed the field army at the Battle of Nineveh. Khosrow II was deposed and executed two months after that.
For the city, 626 made the reputation of its defences. The next besieger to attempt the same combination of walls and water was the Umayyad caliphate in 717–718, and it failed against the same pairing of Theodosian masonry and a fleet that would not let the sea be used.