Rome under barbarian and Byzantine rule

The fall of the Western Roman Empire made little difference to Rome. Odoacer and then the Ostrogoths continued, like the last emperors, to rule Italy from Ravenna. Meanwhile, the Senate, even though long since stripped of wider powers, continued to administer Rome itself, and the Pope usually came from a senatorial family. This situation continued until the Eastern Roman Empire, under Justinian I, captured the city in 536.

In 546, the Ostrogoths under Totila recaptured and sacked the city. The Byzantine general Belisarius recaptured Rome but the Ostrogoths took it again in 549. Belisarius was replaced by Narses, who captured Rome from the Ostrogoths for good in 552. It was as a result of the constant war around it in the 530s and 540s that Rome at last fell into total disrepair, becoming nearly abandoned and desolate.

Eastern Roman Emperor Justinian I (reigned 527–565) granted Rome subsidies for the maintenance of public buildings, aqueducts and bridges - though, being mostly drawn from an Italy impoverished by the recent wars, these were not always fully sufficient. He also styled himself the patron of its remaining scholars, orators, physicians and lawyers in the stated hope that in time more youths would seek for a better education. After the wars, the Senate was in theory restored, but under the supervision of a prefect and other officials appointed by and responsible to the Byzantine authorities in Ravenna.

However, the Pope was now one of the leading religious figures in the entire Byzantine Empire and effectively more powerful locally than either the remaining senators or local Byzantine officials. In practice, local power in Rome devolved to the Pope and, over the next few decades, both much of the remaining possessions of the senatorial aristocracy and the local Byzantine administration in Rome were absorbed by the Church.

The reign of Justinian's nephew and successor Justin II (reigned 565–578) would see the invasion of the Lombards under Alboin (568). By capturing the regions of Benevento, Lombardy, Piedmont, Spoleto and Tuscany, the invaders effectively restricted imperial authority to small islands of land surrounding a number of coastal cities, including Ravenna, Naples and Rome. The one inland city continuing under Byzantine control was Perugia, which provided a repeatedly threatened overland link between Rome and Ravenna. In 578 and again in 580, the Senate, in its last recorded acts, had to ask for the support of Tiberius II Constantine (reigned 578–582) against the approaching dukes, Faroald of Spoleto and Zotto of Benevento.

Maurice (reigned 582–602) added a new factor in the continuing conflict by creating an alliance with Childebert II of Austrasia (reigned 575–595). The armies of the Frankish King invaded the Lombard territories in 584, 585, 588 and 590. Rome had suffered badly from a disastrous flood of the Tiber in 589, followed by a plague in 590. The later is notable for the legend of the angel seen, while the newly elected Pope Gregory I (term 590‑604) was passing in procession by Hadrian's Tomb, to hover over the building and to sheathe his flaming sword as a sign that the pestilence was about to cease. But the city was safe from capture at least.

Agilulf, however, the new Lombard King (reigned 591 to c. 616), managed to secure peace with Childebert, reorganized his territories and resumed activities against both Naples and Rome by 592. With the Emperor preoccupied with wars in the eastern borders and the various succeeding Exarchs unable to secure Rome from invasion, Gregory took a personal initiative of starting negotiations for a peace treaty. It was completed during the autumn of 598 and was only after recognized by Maurice. But it would last till the end of his reign.

The position of the Patriarch of Rome was further strengthened under the usurper Phocas (reigned 602–610). Phocas recognized their primacy over that of the Patriarch of Constantinople and even decreed Pope Boniface III (607) to be "the head of all the Churches".

During the seventh century, an influx of both Byzantine officials and churchmen from elsewhere in the empire made both the local lay aristocracy and Church leadership largely Greek-speaking. However, the strong Byzantine cultural influence did not always lead to political harmony between Rome and Constantinople. In the controversy over Monothelitism, popes found themselves under severe pressure (sometimes amounting to physical force) when they failed to keep in step with Constantinople's shifting theological positions. In 653, Pope Martin I was deported to Constantinople and, after a show trial, exiled to the Crimea, where he died.

Then, in 663, Rome had its first imperial visit for two centuries, by Constans II - its worst disaster since the Gothic Wars when the emperor proceeded to strip Rome of metal, including from buildings and statues, to provide materials for armaments to use against the Saracens. However, for the next half-century, despite further tensions, Rome and the Papacy continued to prefer continued Byzantine rule - in part because the alternative was Lombard rule, and in part because Rome's food was largely coming from Papal estates elsewhere in the Empire, particularly Sicily.

However, in 727, Pope Gregory II refused to accept the decrees of Emperor Leo III, establishing iconoclasm. Leo proceeded, unsuccessfully, to impose iconoclasm on Rome by military force and then confiscated the Papal estates in Sicily and transferred areas previously ecclesiastically under the Pope but still under Byzantine control to the Patriarch of Constantinople. In effect, Rome had been expelled from the Byzantine Empire.

This left Rome reliant purely on its own local forces to protect itself against Lombard encroachment - sometimes now, indeed, encouraged by the Byzantines. Other protectors were now needed - and finally, in 753, Pope Stephen III induced Pippin the Younger, king of the Franks, to attack the Lombards on the Papacy's behalf.

In the 9th century, Pope Leo IV commisioned the construction of a wall around an area on the opposite side of the Tiber from the seven hills of Rome, which has since been called the Leonine City.