It is also probable that both inclination and timing militated against a Gothic- Roman melding. As regards inclination, we have seen that several Gothic elites stood against too rapid a cultural shift to Romanness, or were opposed to it altogether, and continued to view Italy as just another region in which to camp out, requisition supplies, and command subject populations from a distance. We must not forget that Theodoric was unique in the long amount of time he had spent in Constantinople, gaining a desire for thorough Romanization, as well as a comprehension of its meaning. In terms of timing, Theodoric was not even allowed the space of his own lifetime to preside over the process he is said to have wanted. By the 510s, pressures from the Frankish north and growing Byzantine disquietude made the whole program precarious. Simply put, by the mid 530s, the process of Gothic-Roman commingling had not been allowed to proceed far enough to gather its own momentum.
In such circumstances, one would expect a rapid, sustainable Eastern Roman victory under Justinian's powerful armies and generals. So, why was the Roman return so short-lived? Some of it relates to Justinian's forces and command structure. Though overwhelming at first, Roman armies suffered severe limitations, in manpower in particular. Justinian's wife Theodora was distrustful of anyone potentially a threat to her husband's--and her own-- status. Thus, the daring general Belisarius was only allowed 7,500 men to take Italy, as opposed to the 15,000 he had had in North Africa. In this condition, sustained siege warfare and combat-related attrition had a debilitating effect on Roman advances. Further, mutual dislike among Roman generals, such as Belisarius and Narses, as well as a later tendency of field commanders not to obey the former after he had been temporarily disgraced in the 540s by Theodora, made adequate command structures difficult.
Additionally, the significance of the Sassanian irruption in the 540s was paramount. The Eastern Roman state simply did not have the resources or trusted tacticians at this point to handle simultaneous hard-pressing military challenges on two widely-spaced fronts. This is especially the case when we consider that the new Persian wars were not simply massive raiding expeditions as before. Pushed westwards by Turkic tribes nearly off the Iranian plateau, Sassanians were now geared to permanent conquest of Roman lands. Preventing this would require all available Roman resources. Belisarius was simply unavailable in the West, as Persian fighting demanded all of his attention. On perhaps an even higher level of significance was the epidemic that struck all urban centers in the larger Mediterranean basin from 542 onwards. With perhaps a 33% mortality rate affecting cities and settled peasants the most, it devastated the demographic basis of the Roman state, sapping its material resources. All this was at the same time as Bulgar invasions into Byzantium's north.
This is not to discount the Italians and Greeks themselves. Justinian's ambitions for reconquest were by no means shared by his soldiers, subjects, or Italians supposedly awaiting his salvation. While perhaps popular during the first quick thrusts, campaigns in Italy became increasingly difficult. This meant ever-mounting taxes for Byzantines as well as Italians in areas subject to Imperial control. Furthermore, much of the Western Roman aristocracy had become committed to the Gothic cause in the preceding fifty years, continuing to help them on pain of death for suspected treason. Boethius is a perfect example, as he was executed in 527 by Goths suspecting traitorous communication with Constantinople. As one of the last classical philosophers in the West, his execution has given the Goths a particularly bad name. Of course, Cassiodorus continued to work for his Gothic masters, retiring later to a monastery he founded. More basically, however, by the 520s, Italy was beginning to enjoy its first full generation of relative peace. The Roman revanche from the East wrecked this peace. Major urban areas only beginning to recover, such as Rome, were sacked multiple times, as were Milan and Ravenna. Increasingly, Byzantine forces were as materially exacting from their Roman brothers as had been the Visigoths or Vandals before them. Thus, by the 550s, there were few in Italy feeling better off for Byzantine presence. The Pope, of course, was less than elated. Though often looking to Eastern Rome as patron and protector, previous years of doctrinal schism had weakened the goodwill. Further, Imperial commanders in Ravenna who were unavailable to protect the Papacy, yet financially demanding and politically restrictive, offered little to the Church it could not hope to gain from an arrangement with Lombard kings. This in fact is what Gregory I did in the 590s.
This brings us to our final question regarding the Lombard impact. Looked at in retrospect, while the 540s may have presented a return to Roman unity in the Mediterranean basin, the non-stop ravages of the Roman-Gothic wars from 535-52, followed just fifteen years later by the Lombard invasion, signaled the transition from the Late Antique to the Dark Ages for Italy, as well as Byzantium. The destruction of Justinian's wars was unparalleled in previous times. Compared to this, however, the Lombard invasion was catastrophic. They were the most unreconstructed Barbarians to date, with absolutely no taste for preserving Roman society. Lombards came in extremely large numbers, tipping the demographic balance in Barbarian favor unlike before with Visigoths or Ostrogoths. And, while they did settle parts of the peninsula intensely, they spent the first generation--570-600 at least--in unremitted plunder. This had irreversible consequences in ecological terms for Italy and southern Gaul. Regarding the latter, inland cities reverted to towns because they were cut off both from denuded countryside as well as from the Mediterranean coast, its trade, and culture. In Italy, remaining landowners fled in large numbers for coastal areas, depriving cities of wealth and vitality. The old Roman administrative structure and personnel were eliminated permanently, with only Byzantine outposts, Lombard duchies, and Papal possessions remaining. The countryside was abandoned by defenseless peasants, who fled to the mountain villages. It is from this time that the ancient terrace system of agriculture was perforce abandoned, both in Italy and Balkan areas afflicted by Slavs and Bulgars. In the ensuing generations, terraces left untended due to Lombard ravaging or plague-related mortality could not stop rains from causing continued erosion. Alluvial deposits called youngerfill swept down from mountains and corrupted previously fertile soil. From the 580s-620s, then, we can locate the onset of the Dark Ages throughout the Mediterranean.