At the turn of the VIIth century, Rome became a border city : while under civilian authority ever since the Roman Empire, threatened and besieged by Lombards twice in the late VIth century it was garrisoned by soldiers initially raised by the pope (who began to assume more political authority in this period, would it be only dealing with Lombards and neighboring bishoprics in disarray before the former's' conquests).

We don't know when exactly, but this garrison was probably headed by a duke at this point, assuming military command over the remaining territories of the former provinces of Urbicaria in Central Italy with its local garrisons and castra; as well as, eventually its broad administrative authority being thus in opposition to the pontifical authority who more or less inherited the functions of the urban prefecture.

While the overall authority was militarized, and as Italian defense was to be ensured by local recruitment, a "militarized landholding" emerged in the peninsula; with possession coming from the largess and contacts with authorities (especially the Roman church, which as the biggest land possessor in the region, could "trust" lands to their new fides,) and with a status which was ensured and displayed trough the distribution of titles and honors from the pope, the duke and the exarch. By the VIIIth century, elites were Italian, strongly tied to regional and local communities, and related to Italian authorities more than a remote Constantinople : eventually, proximity of one's court, prestige and capacity of redistribution was one of the decivive factors that decided the political upcoming of late Imperial Italy, the pope having an obvious advantage over the dukes (which are scarcely attested for in the late VIIth century) but as well towards the exarch of Ravenna in what became his "home turf".

When it come to the emperor, he was seen in Rome as a flawed protector : he was unmistakably the secular head of Christiendom, but one that overstep imperial prerogatives when it came to religious organization and dogmatisation in a period where Roman papacy stressed its primacy in the wake of clergymen fleeing the invaded eastern and Balkan provinces and sheltering in Italy.

What was eventually got over with under Justinian's reign became unacceptable. Justinian II's less than subtle decision to remove Sergius out of Rome in 693 furthermore made clear that Italian nobles and armies were rather loyal to their own regional interests and a pope perceived as figure-heading these against Constantinople and ready to defend the pope against the emperor; something that happened again in 701.

In the early VIIIth century, popes had an obvious prestige and power that was still more completed and negotiated by imperial, exarchial and ducal authority than really contradictory : the pope Constantine was recieved in Constantinople in quasi-imperial honors and was granted religious jurisdiction over Central Italy, including Ravenna, which created some tensions giving the regional tendencies existing as well in other Italian duchies. The Imperial anarchy only furthered this pontifical importance in conjunction of other, lay, officials in a political compromise (popes tolerating the less than orthodox practices of various emperors).

But this de facto compromise was utterly damaged by both the renewal of Lombard attacks on Imperial Italy, and the iconoclast crisis. As emperors had to deal with the siege of their capital and wars against Arabs, in Italy the Imperial territory was slowly, but surely, eaten away in spite of disorganized local efforts of counter-attack and critically diplomacy but as well significant opportunism from both Italians and Lombards.
The only clearly steady figure in the region was for most, the pope, whom sacerdotal authority was acknowledged in western and north-western kingdoms, allowing them to deal directly with Lombard according their own interest and perspective. Constantinople attempted to have the dukes of Rome relaying their politics, but without much effect and at least a duke took the pontifical side and was unable to be removed by the emperor. Still, while the Duchy of Rome was mostly autonomous, we'd rather see there not as much velleities of independence but fierce displays of orthodoxy (against monothelism) and regionalism.

Leo III's policy was the proverbial nail in the coffin there, as popes refused to pay taxes levied on church property; It led to a complex affair with the dukes and the new exarch attempting to force pontifical obedience either by military defeat or by murder and coup. It utterly failed, first due to difficulties raising troops in Ravenna and Pentapolis, and then as a new duke was sent with troops due to both Italian revolt comparable to 693 and Lombard support to the pontifical cause. After 725, it was clear that the emperor could barely be obeyed in several matters in the exarchate, and not at all in the Duchy of Rome. It could have been salvaged from an Italian point of view (as popes regularly tried to do, preventing things to go too far) if not for the Iconoclast crisis.

While a not-that-orthodox emperor could be tolerated if he managed to defend Italy and the pope; an outright heterodox emperor unable to prevent Lombards gnawing at the bones of the duchies was a cardinal sin of its own. In 727, the revolt was general in Central Italy, proclaiming to defend the pope against the emperor, proclaiming their own dukes, and the Exarch being attacked by Lombards displaying themselves as defenders of Rome and orthodoxy. For all practical purposes, the Pope and the Duchy of Rome, now under its control, were independent, although the pope attempted to paint themselves still within the empire until the mid-VIIIth century but at their conditions (imperial edicts and rules being applied or not at their discretion).

From there, the history of the duchy until an alliance with Carolingian dynasty was firmly formalized is mostly an history of playing the balance game : the protection of Lituprand was uneasy and the Exarch made no secret of his ambition to bring Rome back to his obedience. After a failed alliance with the dukes of Spoleto and Benevent (which almost led to an alliance between Pavia and Ravenna), Gregorius II had to accept being a semi-protectorate of Lombards (trying some good gestue to Constantinople to compensate), sparing him the imposition of iconoclasm and imperial decrees; which allowed Rome to maintain his prestige in Christiendom as Leo doubled-down on the abolition of icons, critically as the whole of Imperial Italy concieved itself as drastically distinct from eastern provinces, leaving only the pope in Central Italy as a legitim figure, one that was able to claim Ravenna as part of the "holy republic" basically both Orthodox and Imperial interests in Italy against both Imperials and Lombards respectively; something that was eventually obtained trough the Donation of Pepin.

In short, what happened is that the Duchy of Rome knew a similar evolution than the rest of Imperial duchies in Italy; but the pontifical authority cementing both a Roman regionalism and Italian particularism, thanks to its defense of orthodoxy against the emperors, its capacity to bind to Rome militarized landholders and thus defend itself. Not that they searched or planned independence, but regardless of their intention, they "led" it to quote Thomas Noble.

- Le duché byzantin de Rome. Origine, durée et extension géographique; Bernard Bavant; 1979

- The Republic of St. Peter: The Birth of the Papal State, 680-825;Thomas F. X. Noble ; 1984