INTRODUCTION
Historians usually agree on considering October 10th 618 as the official date when the Eastern Roman Empire (also called Byzantine Empire) ceased to exist.
The Sassanid troops had made their way into the city of Constantinople only few days before [1] and the remaining Byzantine officers died when they desperately tried to fight the invaders. The last Eastern Roman Emperor, Heraclius, had tried to flee the city just before the Sassanids put the city under siege, but his fleet was intercepted at some unknown point in the Aegean Sea. His fate is still unknown today, but it’s unlikely that he could have survived for long, either imprisoned or not.
The walls of Constantinople, captured by the Sassanids in 618.
On October 10th, the Sassanid Emperor Khosrau II declared Constantinople and its surrounding area as a new part of his glorious Empire, wrapping up the last military action of the Sassanid-Byzantine War (602-618). The Persians had previously conquered and incorporated the former Byzantine provinces of Syria, Egypt and Anatolia, while the Avars and the Slavs had took advantage of the situation for raiding the Byzantine possessions in the Balkans, falling many of them under the expanding Kingdom of the Avars.
When Constantinople fell, the surviving Byzantine effectives that could escape from the Sassanids sheltered in the remaining Byzantine provinces in the West, now isolated territories deprived from the former imperial authority. Prior to the final collapse of the Empire, the war in the East had forced the Byzantines to abandon some Western provinces to their own fate. The last Byzantine strongholds in Southern Spain and Mauritania had been captured by the Visigothic Kingdom in 610, while most of Byzantine Italy had been falling under Lombard rule; Rome and its region (Latium) became an independent entity known as Duchy of Rome(where the Pope acted as Dux himself) after the fall of Ravenna. Other Byzantine enclaves subsisted as independent and isolated entities for a shorter or longer time; some of them established some sort of allegiance to different powers, including post-Byzantine Carthage, but all of them were finally annexed at some point.
After these events, the last Byzantine stronghold that survived the fall of Constantinople was the Exarchate of Africa. By 620, the Exarchate still controlled Northern Africa from Numidia to Tripolitania, as well as the Balearic Islands, Corsica, Sardinia, Sicily, Malta and some territories in Southern Italy. However, most of this power was purely nominal. The Byzantines that managed to escape from the collapse in the East settled in Carthage, the capital of the Exarchate, as well as Eastern Sicily and other Byzantine cities in Southern Italy.
In 621, an officer called Eustatius became the effective ruler of the Exarchate after a local riot between diverse military factions. Officially, the head of the Exarchate was vacant since 617, when the last Exarch died battling a Berber incursion, and most of the territory was in the brink of total anarchy. Eustatius and his fellowmen managed to impose his rule over Carthage, but they could not enforce their authority in many parts of the former territory of the Exarchate, which had started to break apart, as some provinces refused to recognize the legitimacy of Eustatius as their new ruler. Thus, the Balearic Islands, Corsica, Northern Sardinia and Numidia finally fell under the rule of different local powers, and Eustatius could not make anything to prevent this.
Eustatius reorganized the remnants of the former Byzantine troops and fought the militias that still tried to challenge his power. In 622, Eustatius requested Pope Boniface V to recognize him as the new Roman Emperor, but the Pope refused to do so; however, he accepted to recognize Eustatius as Supreme Consul of Carthage and thus, he legitimated his rule over the former Exarchate, which was renamed as Consulate of Carthage shortly after.
In 623, the Lombards conquered the semi-independent city of Naples, disconnecting Rome from the Consulate. The Pope requested military aid from the Consulate in order to secure the borders of the Duchy; however, the troops of the Consulate failed to expel the Lombards from Naples, and Rome became increasingly threatened by the Lombard advance from its Southern side.
Pope Boniface V realized that the Consulate was not able to provide the necessary aid and thus Rome would eventually fall under the Lombard advance sooner than later. It become pretty evident that Rome needed a different external power that could protect it against the Lombards, as Byzantium no longer existed and its heir entity, the Consulate, was still too weak for defending Rome from barbaric invasions.
The Pope and his council debated the available options. Byzantines excluded, there were only two foreign powers at that time that had adopted the Nicene Christianity: the Franks and the Visigoths. Other powers were completely ruled out, as the Papacy completely distrusted any power that was following an alternative belief.
The Franks would be the preferable option, as they had adopted the Nicene Christianity long time ago and they had a powerful army. However, the Franks were involved in a bloody civil war just at that moment, confronting the rival realms of Neustria and Austrasia [2]. So, the political situation in Francia prevented that the Franks could help Rome by that moment.
The second option was the Visigoths of Hispania. Visigoths had adopted Nicene Christianity recently (589) and thus, they were perceived by Rome to be not as trustable as Franks, not to say that their military power was more humble. The council of Rome was reluctant to look for their help and preferred to wait for a viable solution with the Franks; however, the Lombard advance was faster than expected, and Rome realized that they could not wait for long.
Liuva II, Visigothic King of Hispania.
Thus, in May 624 a Papal delegation was finally sent to the Visigoth port city of Tarraco, where King Liuva II [3] and other Visigothic noblemen were waiting for them…
[1] First PoD: After the fall of Chalcedon in 617, Sassanids are able to launch an attack and siege of Constantinople without peace negotiations. OTL the Persians had to withdraw troops from the area for supporting the invasion of Egypt, after a delay caused by failed peace negotiations.
[2] Second PoD: Chlothar II is not able to solve the tensions between the Frankish realms of Neustria and Austrasia and a civil war between them breaks out. OTL Chlothar II appointed his son Dagobert as King of Austrasia and avoided the crisis.
[3] Third PoD: Liuva II survives the coup of Witteric, who is executed. Liuva II consolidates the Catholic Kingdom of Toledo and avoids two decades of continued regicide and political instability. OTL Witteric deposed Liuva II in 603 and opened a period of great instability in the Visigothic Kingdom.
Map of the Consulate of Carthage and the Duchy of Rome:
In yellow, the area controlled by Eustatius in 624. The areas in pale light yellow officially belong to the Consulate, but they are no longer controlled by it after 621; the areas in light green are former Byzantine territories lost to the Lombards after 618.