Few topics I was thinking about:
1- Reduction of Roman Legions size
2- Increase in the size of entire military during late 3rd and 4th century
3- Military reforms of Diocletian and Constantine
4- Disbandment of the Praetorian Guard
5- Comitateses and Limitanei
6- Role of religion
7- Barbarisation
8- Overall issues that were and weren't improved
1- It was needed. Five thousand men formations are great for field campaigns and great battles but when most of the fighting is done in small skirmishes the size of the legion would work against itself.
With the 1000 men legion the Roman army gained flexibility and its ability to fight off small raiding parties improved.
2- This point is problematic. As someone will probably point out the two main lines of thought on the size is either the Larger Imperial Army or the Smaller Imperial Army.
I don't known which side is right but what I know is that until Diocletian/Constantine the Army had a tendency to increase in size and that it reach the peak under one of the two Emperors and that from that point on it begun to shrink.
But if the opinion asked is the why it increased during those day, my thought is the reason was civil war.
During the Crisis of the Third Century the Roman Army abandoned many of the weapons and gear that live on the imagination of people today. The gladius was abandoned in favour of the longer spatha, the shield changed from a heavy rectangle to a more light and manoeuvrable oval shape, the pilum was abandoned in favour of lighter darts and the thrusting spear was introduced to the legions and stopped being a weapon of the auxiliary.
The early weapons all required time to get used to.
The gladius is used to stab, and the main instinct of a person when he has a sword is to slash not to stab so it would take time to train the legions in that stile of fighting, while the Spatha is a open quarters slashing weapons.
The Scutum was heavy and demanded strong muscles to be able to hold it for long periods of time during battles, while the new oval shield was lighter and more designed to movement warfare and not the static fight that the romans favoured during the early days.
The pilum was a javelin, that if needed could be used as a spear, but most likely required heavy training to be used to use it effectively and the time it took to construct was, probably, greater than its replacement the Angon and the Plumbatae. Also the spear and the dart are easier weapons to use, don't require much training to get used to it and were lighter and more easier to transport.
All of this indicates that the Roman Army had changed from being a heavily drilled army into a more numbers based forced.
Lower times of training allow more soldiers to be battle-ready compared to those the Early Empire could train in the same amount of time, also the lower number of men per legion wouldn't force the recruits to train the same large formations the Legion of the Early Empire use.
This would allow Emperors, or usurpers, to raise more soldiers in a much smaller window of time and given that the men on the loosing side were usually added to the winning army the size of the field army would keep growing.
Ex: A General declares himself, or his declared by his men, Emperor. His first move is to gain access to gold/silver to pay his troops and to conscript more men into his army so that he can face the Emperor. In this scenario it doesn't matters if the Emperor or the usurper wins for the loosing army will be integrated into the winning side, because extra soldiers were always welcomed by the Emperors and because if they disbanded the men it could lead to instability as they would most likely turn to banditry. Also disbanding the men would give a dangerous sign to his own men, after all if he did that to them what would stop the Emperor to do that to his own men, the logical end, to the legions, would be that the Emperor must be replaced for a more understandable candidate.
3- Before answering this I will public admit my bias against Constantine, not Diocletian tho. Those that read this will know that this is a biased opinion and must take it for his value, and that is to the reader to decide.
Diocletian did what he had to do.
Instead of being the Army the one to serve the Empire, thanks to him, it became the Empire that had to serve the army.
He divided his forces to better protect the Empire, divided the civil and militar command, appointed regional commanders to take care of problem at the lowest level, passed conscription laws that helped the army to recover its lost manpower in the last century, improved militar infrastructures and improved the supply system.
His plans and reforms gave the upper hand back to the Roman armies and, as history showed, his reforms helped the Empire turn his back on his darkest hour.
Basically he was the Emperor the Empire, and the Army needed.
But his reforms also had problems.
His Tetrarchy could only work while a Strong Emperor that could force the other three existed, and while this could be achieved, the other problem was that there was nothing that would stop one of the Emperors to turn on the others.
Now lets get start on Constantine.
He continued Diocletian's work, good move there, abolished the Pretorians, his greatest idea in my opinion, and then he fucked it up by concentrating the armies in the inland cities leaving the borders under the care of the outnumbered Limitanei.
While I understand the reasons why he increased the size of his
comitatus, it was a time of instability and it was dangerous to leave a general with too much soldiers, but transferring border units to integrate them in the mobile army, that was usually stationed deep in the Empire, weakened the borders too much and ruined Diocletian's plans that had been based on strengthening those borders.
P.S. Yes I agree with Zosimus accounts on Constantine and I also agree on his opinion that he weakened the Empire by strengthening his own
comitatus at the expense of the border armies, but like i stated in the beginning this can be considered a bias opinion, I'm aware of it, so you choose the value of it.
4- Needed and, in my opinion, the best decision Constantine ever did. They were over payed, corrupt, had no real use now that Rome had stopped being the true center of Roman Power and they were too dangerous to be left alone because of their history of making their own Emperors.
5- The Comitateses were born out of circumstances.
The Pretorians were too disloyal and the Emperor needed to have an army at his disposal, to fight the usurpers.
They were born out of the Civil wars as the Emperors became more and more enrolled with the army. The days were an Emperor could sit in Rome while his general fought for him were done and now the Emperor was first and foremost a General that needed to make sure that his men were loyal and for that he needed to have a force at his disposal that could make sure they would be kept loyal.
While they were, officially, a mobile army to face off invasions, most of the time they were stationed too inland to act with enough speed. In reality they were basically a giant force of bodyguards.
Limitanei, the unwanted brother of the Comitateses.
As the field armies became more permanent formations and grew in size, less and less men begun to be stationed in the borders. They had to man the forts, scout the frontiers, fight off small scale invasion and do what they could to delay the large invaders until the field armies could advance.
While they represented the largest share of the Army, they were still to few for such a large border. The fact that they were stationed in the borders and had a lower pay, most likely affected their discipline and moral. I also believe that their lower pay was what, probably, made their transformation from professionals to semi-professional to militia soldiers.
They were effective at first but as the borders begun to be more and more harassed the leaks on the border begun to show.
In a book I own, but that I don't have at hand right now, about the Roman Army, the authors defend that the Battle of Adrianople signified the destruction of the Eastern Comitateses and that the Emperors begun to transfer the border forces to the Comitatus. If this is true then it explains the why the Limitanei begun to lower in efficiency.
In the same book the failing of efficiency in the West can be traced to Frigidus. The mobile field army was crushed and the border forces were striped from the frontiers to create new field forces that weakened the frontiers.
6- Won't touch this. I have bias on the religious part, I believe the purges the Pagan/Christian Emperors did only stripped them of valuable manpower.
7- Barbarisation is probably the wrong term.
The Romans had a successful history with using foreign auxiliary and that never weakened their efficiency.
The great problem was the use of large forces of mercenaries.
I once went, with my father, to a lecture where former Portuguese and Spanish officers, mainly from the Ifny war and the Colonial War, would speak of the current world situation. One of the Spanish, a former Brigadier I think, established a parallel between between the usage of contractors to serve in the conflicts and the Roman dependency on foreign mercenaries. He said that when a state favors the usage of private forces for short term gains, will, on the middle-long term, suffer.
Rome proved that.
While the usage of mercenary auxiliary helped Rome short term, in the long term the army begun to be too dependent on them. As the Roman soldiers begun to die more because civil war than foreign war, Emperors begun to relay on employing mercenaries that had no loyalty to him but to money (not that that makes any difference because the soldiers born on the empire were also only loyal to money), because they were cheaper, the state didn't had to invest on their training, and where easily available. The usage of the mercs didn't weakened the efficiency of the army in any way, but created a dependency. And as the Roman born citizens begun to be more and more reluctant to join the army, more mercs had to be recruited, more money had to raised to pay them and when that money didn't appeared land had to be traded away. Less land less taxes, less ability to pay more land as to be given away.
The Empire, like a body, begun to sacrifice its arms and legs to survive, until only the vital parts remained intact. And when there was nothing more to be traded away, they ended up with no army, almost no lands, and with a great number of foreign mercenaries waiting to be payed.
8- The army was efficient and worked well in times of stability.
The greatest issue, that would be hard to fix, was that even in the time of greatest stability there was nothing to stop one General to declare himself Emperor if he had support of a part of the army. Maybe a keeping hostages from the General's family in the Capital could work, but for some not even that would stop them.
But the greatest problem wasn't on the army, it was on the imperial bureaucracy and on civil war.
Corruption was too common, the Emperors had almost no knowledge of the state of the Empire, the Emperors were usually paranoyed and they could even drive the most loyal supporters into enemies, etc... the list is very long.
The armies spent more time fighting Roman forces than fighting foreign enemies, the manpower was destroyed fighting internal enemies, infrastructures were destroyed to deny usurpers their use, which would then be useless when the Emperors needed them to fight external enemies.
There were too many problems and it would be needed a new Five Good Emperors, in the term of five Emperors that didn't had to spend more time fighting insurrections and that would rule more than 5 months, to try to fix them.
