Throughout the 7th century the Eastern Roman Empire was constantly at war with the Western Empire, with Persia, and with itself in the form of rebellions and usurpations. The result was that, although it managed to survive all assaults, by the end of the century the empire had been significantly weakened by the enormous loss of manpower and by the loss of trade.
The Bulgars had until recently dwelt north of the Black Sea as part of a confederation, but with the death of their khan Kubrat, and under increasing pressure from the Khazars, they united under Kubrat's second son Asparukh and began a great migration west. Some Bulgar tribes wished to split from the confederation, but they were cowed into obediance by Asparukh's execution of his brothers and rivals. By 677 the Bulgar Khanate had arrived in Roman Moesia and begun to make a new home.
In 678 the Roman Empire signed a peace treaty with the West, freeing up troops to dislodge the Bulgars. Emperor Constantine IV personally led a large army drawn from all over the Empire, including Slavic allies, and attacked the fledgling Bulgar state. Asparukh gathered his people and withdrew to the island of Peuce in the Danubian delta, where he made his stand.
The Bulgars fortified their position with wooden ramparts. The marshy terrain forced the Romans to attack in small groups, which failed to breach the defences since neither ladders, rams or flames were much use on wet and muddy land.
On the second day of battle Emperor Constantine, who had been suffering from leg pains, left the army to seek medical treatment. Rumours spread through the Roman army that he had fled and no further attacks were mounted that day. Unknown to the Romans, their Sclaveni auxiliaries had come to an arrangement with the Bulgars, and early the next morning the Sclaveni and Bulgars launched a simultaneous attack from both sides against the demoralised Romans.
Unprepared, the Romans were unable to form a battle line and began to rout within minutes. With their sole line of retreat blocked by the Sclaveni, many Romans tried to flee into the swamps where they either drowned or were hunted down and killed. A few units managed to make a last stand on a hill in the middle of the camp, but by midday they too had been cut down. Apart from the Emperor and his bodyguard, and the defecting Sclaveni auxiliaries, only a handful of Roman troops ever managed to make it back home.
News of the battle encouraged rebellions by many Slavic tribes living on Roman territory, and with the destruction of his army Emperor Constantine was unable to suppress them. The next year Asparukh, having formed an alliance with the tribes, invaded Thrace and Macedonia with all his might and conquered the region having faced next to no resistance. The Western Empire, which had been occupying Epirus and the Pelloponese, withdrew its troops in October, allowing the Bulgars to rule supreme there as well.
In 680 Asparukh marched on Constantinople and, after a siege lasting several months, the city capitulated. Constantine escaped to Asia, where he established a new capital at Antioch, but he never forgot his defeat and humiliation. On the 7th July Asparukh was proclaimed Emperor of the Bulgarians and the Romans, and this event is traditionally held to be the foundation of the First Bulgarian Empire.