I think you would need to define "later" to get a relevant answer, but the short one is: your basic premise is flawed.
I'll note that a lot of what I am about to say directly ties into my own research, so I will not go into too much detail (first rights issues and all that). If asked about specific points, I will try and answer in detail.
First of all, let us look at the historiography. History is not independent of the time period in which it is written. You will find that the "problem" of Barbarization mostly starts to gain acceptance in the 19th century in modern history and then is listed as a real problem. It then gained a lot of traction during the later 19th century, coinciding with a lot of military theorists claiming that the strength of a nation lies in the power of its own people, in the power of the citizenry of the predominant race fighting like true patriots for their nations.
So you can see a pretty easy pattern to emerge. Why did the Germans beat Varus at the saltus teutoburgiensis? Because they resisted foreign rule and defended their race from invaders. Why did the Romans fail? Because they hired Germans to do their job instead of being true racial patriots and the dastardly Germans or Romans stabbed each other in the back more than fighting invaders. You couple that with a period of Germanophobia on the side of French/English writers and of the hate of everybody west of the Rhine on the German/northern European side and you easily add up with a pretty racially charged and dominant theory accepted by everyone back then.
Suffice to say, the outlook has changed significantly since then.
Which leads us to the following questions: Was there really Barbarization in the later Roman Empire?
This is the most important one and one that poses an interesting question of terminology. First of all, what is a barbar?
This is not as easy to answer. For example, when the Romans classified people, they did not do so on a racial basis. They did it on geographical and political association. One prominent example is the division into celts/gauls and Germans. When the Romans conquered the Rhine areas they arbitraily decided that everyone who lived on their side was a celt/gaul and everybody on the other side was a German. This was not based on any ethnic or racial trait. We know of the Romans relocating entire tribes, taking them from the German side and then settling them on the celtic side in the same year this distinction was made. We know that people who had been populating both sides of the river and considered themselves one tribe were split up or abitrarily classified as one or the other. That btw is why classifications such as "German tribe" or "Celtic tribe" can only denote political alignment at best and even then there are far too many examples to refute such a strong distinction.
What I am getting at here is that the tribes of antiquity were no united front, nor were allegiances a matter of ethnicity.
One rather easy way to go about this would be to say "everybody whose tribe does not owe direct fealty to Rome is a barbarian". Alright then, but how do you classify a Parthian, coming from the great city of Ctesiphon, being highly educated in several languages and having served in the Imperial bodyguard for 25 years and then having settled as a Veteran in Rome herself, fathering many children who then also served Rome? Is he still considered a Barbarian? (An analogy would be the many hispanic immigrants serving the US Army today - are they american or hispanic?) My point is, just because somebody is from a certain origin does not mean he is not a Roman. Heck, there were plenty of Roman Emperors who were not Roman citizens at first or were of "non-Roman" stock, even during the early 3rd century. For example, Maximinus Thrax was from Thrace, Phillippus Arabs was of originally Arabian decent and one of the most famous dynasty of Rome, the Severans, were descended from Northern African tribesmen. The Emperors Diocletian, Constantine and most of their successors were the descendants of the Illyrian tribes (who originally had to be subdued in year-long and bloody campaigns). Yet all of these somehow are thought of as typical Romans and they themselves believed them to be Romans. Yet while we accept that Emperors might be of barbarian descent and do not question their ancestry nor loyalty to Rome, somehow some people think that a lowly spearman living in Roman territory for most of his live after being recruited as a teenager does not count or has questionable allegiance.
The situation gets even more laughable when you consider that most of these tribes would have had over four-hundred years of direct Roman rule and cultural assimilation. They would have seen their tribesmen become Roman officials and maybe even Emperors for centuries. And then, during the reign of Caracalla, the Constitutio Antoniniana made everybody inside the territories of the Roman Empire de iure a Roman citizen. If you have been a citizen of a nation for over 200 years, how the heck are you not a Roman? It would be as preposterous as starting to count the tens of thousands of soldiers with polish names (and whose ancestors had been "German" citizens since Frederick the Great) who fought in the Wehrmacht not as Germans.
But, for the sake of argument, let us assume that anybody who came from a certain tribe was still a Barbarian. This leads us to the next Question: How numerous were Barbarian units in reality?
This leads us right into a huge problem.
During the Principate (1st and 2nd centuries AD) the Roman Army was split into different categories. We had legions (recruited only from Roman citizens) and auxilia (split into units of Roman-citizens only and non-Romans). Of the over 350.000 troops per annum during that period, roughly half were non-Roman auxilia. This gives us 175.000 Barbarians.
OTOH, the late Roman army only had 65 auxilia palatina, which probably count as only 30-45.000 Barbarians in the field army. (There also are some auxillia units in the provincial armies but their relation to the other units is a minority one as well). We also have a large number of legions bearing barbarian names but that is also not an indicator they were barbarians.
So the main thing to take away from this is that there is the possibility of the army of the 1st and 2nd centuries might have had more non-Romans than the late Roman Army, proportionally speaking.
But it gets even worse for the pro-Barbarization people. Just because a unit has a barbarian name it is not filled with or recruited from Barbarians. Let us take a unit with a barbarian name. If one looks at the actual records of the soldiers and their names on the tombstones, it becomes clear that even in units called "Franks" or "Goths" the number of Barbarian names is in the vast minority, oscillating between 15-40%. This means that even so-called "barbarian" units were majorly filled with Romans. This gets even more serious the longer a unit has been in Roman service and the more it has been transferred. For example, the "Dacians" were at one time transported to Egypt. We have Papyri showing that in the same year they arrived, they started to recruit from the local population to replace losses. So now you have an Egyptian Roman serving in the "Dacian" unit. Quite frankly, the Romans did not care much for race. What they cared for were warm bodies marching under the standards. Their blood, looks or heritage were secondary to ability and willingness to fight.
So you can imagine how in the Late Roman Army, which existed for over 350 years in that command structure and organization, the name of a unit means squat in determining how "barbaric" they were, especially considering units were transferred from one end of the empire to the next. Heck, we know of one unit for example that was transferred from Italy to Gaul, to Britain, to the Balkans, to Persia and then back to Gaul. Within one decade, much of which was spent in hard fighting. I very much doubt this unit was anything but a mixed bag of many recruits from different ethnic heritages at the end of that - and that is supported by many sources. Even if we did not have those sources, the alternative would be pretty comical - imagine Roman officers saying "what, we lost over a third of our men in an ambush of these damned scots? Better wait over a year until the next gothic tribe shows up and provides recruits then. What do you mean, we go against the Germans next summer? Better do it understrength then, we must not dilute the blood of our beloved barbarians with those english Roman citizens. Don't worry, I am sure the Emperor will not totally have our heads when we fail to hold the line due to missing half of our men."
So to summarize - there is no evidence suggesting there was a barbarian majority. We do have evidence for a barbarian minority. But these would be quickly assimilated - Latin was taught from the beginning, simply due to the need of the men understanding orders and, you know, actually being able to talk to the new recruits from whatever province they were currently stationed in.
Which brings us too....A barbarian is a barbarian is a barbarian...is a Roman?
Did somebody who served in the Roman Army consider himself a Barbarian or a Roman? For the enemy it was pretty clear. If you march under the Roman standard, you are a Roman no matter if you have blonde, black or red hair. But how did people born outside of Roman territories view themselves?
First of all, we have to note that for most Germans, Goths etc. the supremacy of Rome was not really in question. Most of them liked the Roman way of life. Rome usually provided a safe haven from trouble like blood feuds, civil war etc. There was a large history of Germans signing up in the Roman Army to escape precisely such troubles. Who do you think the loosing party in a civil war liked more - the people who burnt their homes and killed their friends or Rome which was all too willing to feed and cloth them, as well as pay them good money to fight against the people who killed their friends?
Even moreso, we have a large history of immigration into Roman territory. Even Augustus already accepted German settlers. And why wouldn't an Emperor do this? You could easily (re-)populate land, gain more taxpayers and manpower for the Army. The Romans at times even encouraged such immigration. And the "barbarians" largely accepted it. It is of note that even the goths who defeated the Romans at Adrianople were originally only begging for Land to be settled in and were willing to accept Roman overlordship until corrupt Roman officials starved them and tried to enslave them (or so the sources tell us. Huge shovel of salt here, but the overall point still stands). There is also no record of Germans being proportionally more disloyal. Heck, the ancient sources usually take great pains to mention the loyalty of the German Imperial Guard - which was more loyal than the "pure Roman" Praetorian Guard.
This brings us back to the individual men. Suppose you had served your time and were now a veteran. What choice would you make? Settle in Roman territory with full honours (and with your wife and kids), enjoy the comforts of civilization like free food for life, access to political life, daily hot baths in the Thermae or go back to cold, foresty Germany where chances are nobody really knows you anymore? If you were discharged in sunny Egypt, do you really want to take a trip across the entire known world to go back to the swamp? In fact we know of very few veterans who did not settle in Roman territory at the end of their service. I only know of one documented archeological evidence. The vast majority settled in Roman territory near their comrades in arms. We also know of cases where such veterans took up arms and defended Roman territory from invaders, even former tribesmen of theirs. At this point, I see little value in discounting the massive integration machine the Roman Army was in favor of calling people like that Barbarians.
And let us not forget that to become a Roman soldier provided a lot of upward mobility, provided one survived of course. Heck, your son which you fathered with a camp follower might even become emperor. Enticing possibilities for farm hands or farmers, right?
There is a problem however when considering Barbarian nobility. Essentially, many of these guys already had a "Roman" education even while being barbarian. They mostly would be speaking Latin already. Still, even a lot of those became Roman officers due to political issues at home or because the rewards were still substantial. For example, Nevitta, a Frank, became consul and I am sure I do not have to list the long list of dominant "barbarian" Generals. However, most of those were already romanized to the core and had nothing but the goal of marrying into noble Roman families or the Imperial household.
Anti-German backlash and loyalty trouble only seemed to have occurred when whole tribes were permitted to keep their political structure intact in the fifth century and Germanic tribes became more of a widespread problem. However, note that this is only occurring 1.5 centuries after "barbarization" was already in full swing and the most loyal General of that period was of Barbarian Origin (Stilicho). In fact, Barbarian officers and men tried to become as Roman as possible even during that time.
So IMO the whole issue of Barbarization is a non-issue. The Romans clearly thought there were no large problems to recruiting Barbarians, who often proved themselves to be loyal, dependable and elite soldiers. And the Barbarians themselves were oftentimes all too glad to be Romans, and to be accepted as such.
Two salient examples: When Julian defeated the Franks, his elite regiments were commanded by Nevitta, a Frank himself. He became Consul of Rome. When the Alamanni, a coalition of germanic tribes, invaded Gaul in 365, they were opposed by the Roman General Dagalaifus, who himself was a "German". He became Consul of Rome the following year. Were they acting as Barbarians or as a Roman officers? In both cases Germans made up a portion of the army that beat back the German invaders. Were they acting as Roman soldiers or Barbarians? The answer should be clear.
Now, to the specific questions:
mr friendly guy wrote:
1. What did they do to try and reverse this trend?
Why would they do so, considering that "barbarians" had been recruited by the empire for over 400 years and some of their best soldiers would be Barbarians? Also note that the Romans had manpower issues.
2. Why couldn't they succeed in reversing the trend? Or to put it another way, why did it become harder for the Romans to recruit native born Romans into the mix?
- There is no evidence that the Romans really had to
- The Romans had very high recruiting standards resulting in less than 2% of the Empire's population being able to meet those. These standards were lowered in the later centuries but serving as a soldier was more attractive for the frontier armies, not the field armies (bad risk/reward ratio, service far away from home, high mortality, better chances for economic advancement in the local economies). As a result Barbarians became more important to the field armies as they were able to provide more recruits - and even better motivated ones.
- Every recruit you hire and romanize is one less Barbarian to worry about. If he has healthy sons, you even get bonus soldiers out of it.
3. Was this just confined to the Western Roman Empire? Or did the Eastern empire also experience similar things, but to a lesser extent? In which case, why wasn't the Eastern Empire "barbarianised" to the same extent?
There is no evidence for lesser or higher barbarization on part of the Eastern Empire, except for the last fifty years of the Western roman empire. During that period the following happened:
- The regular roman Army structure pretty much collapsed in the west after the death of Aetius 454 and the events that followed it.
- Barbarian tribes were now recruited wholesale, under their own commanders, unit structure and laws. This means the process of integration was not happening that well anymore, though mixed units still existed. In the east only mixed units might have been recruited and in the west they might still have been in the majority. But besides these there were also the germanic tribes recruited wholesale, which were much harder to command and often were a pest.
- As the entire western Empire was overrun by Germans, central Roman authority disappeared and so did the Roman control and Roman Army. The East was not overrun, ergo it maintained its previous structure.
However, please note that we are talking about a very small window of collapse when compared to the centuries of success (20+ years compared to over 400 years of success of hiring "barbarians"). But this window is still very large - consider that even with "barbarians" being predominant the Romans still managed to survive for 20 years. Which is a very long time to survive as an Empire under constant attacks. No other empire in European history has had a total collapse of the central structure and still managed to survive for over 20 years, in no small part due to Barbarians fighting for the empire.
I hope this answers the questions - I realize the massive essay above might be tl,dr but I felt it necessary to clarify a few things.