One theory I have read:
An important factor with regards to manpower was the growing autonomy of local landowners. As defense against both bandits and foreign raiders became the responsibility of a local lord, his insentive to keep able bodied men at home grew. When asked by imperial officials if there were available men it was then better to lie. As the emperor failed to hold the frontiers and keep the peace, available manpower shifted from imperial to local service.
Yes, that‘s quite en extended opinion. But in my opinion it‘s quite simplistic and unaware of what was the social, fiscal and political reality of the Roman empire.
Landed potentates had always enjoyed an immense degree of autonomy in the Roman empire, it was nothing new in the IV or V centuries CE. What happened from the III century onwards was that in the West (or at least in substantial parts of the western empire) these landed magnates became separate from city life, when cities were the building blocks of the Roman empire.
The High Empire (the Principate) can best be described as a confederation of cities under the leadership of Rome, the same that Rome had done with the cities of Italy during its initial wave of conquests. In Italy, Sicily, parts of North Africa and the East this was an easy task, because a dense network of cities already existed. These cities were coerced by Rome to become Roman subjects, under several ad hoc arrangements, from a city being considered a Roman ally (like for example Aphrodisias in Caria, Delos or Rhodes), to being put under supervision of Roman magistrates, but in all cases local institutions and laws were maintained. These subject cities had to acknowledge the leadership of the Senate and the people of Rome in the person of their magistrates (they had to obey them), they had to pay a tribute to Rome (which varied depending on the case of each city and in which circumstances it had become subjected to Rome), acknowledge the superiority of Roman law (their authorities had no legal jurisdiction over Roman citizens) and had to help Rome in case of war, if Rome asked them so.
The exception were the cities which dared to resist and were destroyed, like Carthage or Corinth, but in these cases the Romans rebuilt them after some time as Roman colonies, assigning them large territories carved out from Roman public lands (
ager publicus).
Thus, the whole apparatus of Roman administration was dependent on cities. Each province was subdivided into
territoria, each of whom which was led by a city. In the case of some Greek provinces, the most predominant among the cities in a province received the title of
metropoleis (that did not necessarily imply that the Roman governor lived there, though) and it was there where the council of provincial sities took place (a gathering of delegates from the city councils (
boule) of each city. And each city, if it had the rank of a “Roman ally“ had even the privilege of sending its own embassies to Rome, bypassing the authority of the Roman governor.
As you can image, the problem came when the Romans conquered areas where no substantial cities existed, like most of Spain, Gaul and North Africa west of Tunis, Britain, and the Rhenish and Danubian provinces. Here it was the Romans who had to create cities
ex novo, in order to be able to administrate these territories. The best known case is that of Gaul, where the Roman administrative structure was set up during the long reign of Augustus. Augustus built a new city in the territory of each Gaulish tribe (for example,
Lutetia Parisiorum in the lands of the
Parisii), and so it transformed (by decree) each tribal land area into an urban
territorium, and an important part of the establishment of Roman rule in the province was convincing the old tribal leadership to take residence in the new cities and adopt a Roman way of life.
As these new cities had no real urban laws or traditions to rule themselves, Augustus had to create them either as Roman
coloniae or
municipia, and whenever possible he settled them with either Italian colonists or veterans from his legions (although the overwhelming majority of these settlements were made in Italy, southern Gaul and Spain, like at
Emerita Augusta,
Caesaraugusta,
Asturica Augusta, etc.).
This makes the question arise of how “real“ were these cities. Did they really fulfill the functions that a city is expected to fulfill (commerce, manufacturing, etc.) or were they just glorified showcases of Roman propaganda, where the local Romanized elites (and the emperor at times) spent ridiculous amounts of money in unnecessary public buildings?. For example, the amphitheaters in some western cities had a seating capacity larger than the urban population (and one should take into account that slaves could not attend the spectacles).
Recent archaeological research has showed that already in the late II century CE a process of contraction and downsizing in the cites of the western empire had begun in some areas, especially in Gaul. The start of this process coincides with the Antonine plague, but it can not be related to any situation of war or insecurity, because with the exception of the civil war between Septimius Severus and Clodius Albinus Gaul was completely at peace at this time. And the Antonine plague affected all of the empire, and many areas recovered from it in a period of a few decades, so in itself it‘s not a satisfactory explanation either.
It could be put in relation too with the trend, which began already in the late I century CE, of emperors meddling increasingly in urban government. One of the first examples can be found in the surviving letters from Pliny the Younger to Trajan. Pliny was governor of the rich province of Bithynia et Pontus, and in it Pliny informs Trajan of a situation in his province that seems to have been quite common at the time. The two largest cities in the province, Nicaea and Nicomedia, were always in a fierce competition with each other, and this time both cities had embarked in such grandiose building plans that both municipal councils were on the verge of bankruptcy. Pliny had intervened and taken control of municipal finances in both cities, slashing completely the funding for unnecesary new lavish buildings, and Trajan approved his actions.
As the II century went by, epigraphical evidence shows more and more examples of imperial functionaries (
procuratores) sent to the provinces by the emperor with the precise purpose of fiscalizing the public spending of the cities in the province. and this development seems to have been very badly received by the provincial landed aristocracy almost everywhere (except in cities which were already in financial trouble, which actually welcomed the intervention). This development seems to have intensified under Marcus Aurelius and his successors, when the Roman
fiscus began to experience growing difficulties. Given the fiscal problems that plagued the emperors beginning with Septimius Severus, it‘s quite logical that they decided to tighten the screws of the urban aristocracies and to ensure that “profligate spending“ was cut back in order to ensure that more money would flow to the imperial treasury.
Today scholars believe that it was this situation that caused the almost total stop in public urban building in most of the western empire between the II and III centuries, and which caused the former urban aristocracies to retreat from urban life. But what‘s interesting to notice again is that this process also happened in the eastern empire and northern Africa, and there the results were very different. There was a certain stop in public building activity, but it did not happen everywhere (Thysdrus in Tunis still built a colossal amphitheater in the III century, and Hierapolis in Phrygia kept up a lavish public building program). So, the real puzzle is why did it happen differently in Gaul, Spain, Illyricum and the Danubian provinces.
In my opinion, the answer must lay in the fact that urban aristocracies in the east and west were very different. Eastern urban elites possessed landed estates, but most of them still lived in cities and kept urban life and institutions going on. Most of them had also commercial interests, and so living in commercial cities was essential. In contrast, the western aristocracy, beginning with the Roman Senate, was essentially a landed aristocracy, like in medieval Europe. In the High Empire they lived in cities because that was the “civilized“ way of life (the very word “civilized“ comes from the Latin word for “city“), but not because they really needed it. When living in cities became unattractive due to the changed circumstances of the III century, they just went back to living in their estates, which were after all the real source of their wealth, and without these elites, the western cities struggled to survive, and with it the whole Roman administrative and fiscal apparatus.
It“s startling to notice that although the eastern empire was altogether richer than the western part, archaeology has found nowhere in the east such luxurious
villae like the ones that were built in the western empire during the IV century CE. Apart from the obvious case of Rome, splendid
villae (some of them even rivaling the imperial ones in central Italy) have been found in Sicily, Africa, Spain, Gaul and even Britain. Some of them, like the one at Chiragan near Toulouse or the so-called “palace of Maximian“ near Cordoba are so immense and luxurious that some archaeologists believe they had been imperial residences, or at least part of imperial estates.
So, a landed aristocracy enjoying large autonomy was nothing new by the late IV century CE, at least in the West. It had been the current state of affairs for at least the last 200 years by then, and more if you take into account the role of the Senate. Before its ranks were enlarged to ridiculous levels during the IV century (by then the Senate had lost any kind of real political power), the 600 men sitting at the Curia in the Roman Forum were the largest landowners in the Roman empire (after the emperor himself) and they were all exempt from direct taxes, a fiscal privilege that continued during the IV and V centuries. By the late IV century CE, most land in the western empire must‘ve been either owned by the emperor, or owned by senators, or by the Church (which by this time also enjoyed the same privileged as the senators). This means that the emperors must‘ve gained very little money from direct taxes (the land tax for obvius reasons, and the poll tax because unfree people -a legal cathegory that included not only slaves- paid far less than a free person). The other source of income were indirect taxes, but if there‘s one sure trend in archaeology in the late Roman empire in the west it‘s the slow but continued decrease in trade, again with the exception of North Africa. But even in the case of North Africa, there were further problems, because African traders were charged with the food supply of the city of Rome, and the Roman state usually paid them with ... tax exemptions. In my opinion, if there was some reason for the troubles of the western empire in the late IV and V centuries CE, the reason must be found in the social and fiscal situation, which was the logical culmination of a very long historical process. The separation of the two halves of the empire at that critical moment in time probably made things much worse, because the western empire was deprived from the financial help of the eastern empire, which still had a functional fiscal system in place.
