Realizing that staying as solely a history major in university I decided to expand myself and enter into the world of Ancient studies, a decision that has been one of greatest of my life thus far. As my time and interest in this field grew, I quickly became fully immersed in everything Ancient and eventually settled on my era of academic focus: the Late Romand and Eraly Byzantine Era. One of the more intriguing themes to me was the evolution and changes that the Roman army underwent over these four hundred years, and over just decades as well. In order to better understand and answer my questions on this theme, I wrote an upper-year research paper on this which can be read below.
From Then to Now: The Differences Between the Armies of Diocletian and Theodosius I
If there is one constant throughout the entire history of the Roman state, it was that of evolution. From politics, social issues, and economics, these and other elements of Rome evolved to match the outside world. However, despite these important individual elements, the single most important element was that of the Roman military. Throughout its 2,206 year history as a state, Rome experienced every type of warfare over the course of this extensive time period. Focusing on a smaller segment of its military history, one of the most important changes to the Roman Army happened between 284 and 380 CE. This segment witnessed the decline and ultimate removal of the legionary style army that made the Roman Principate nearly invincible, and its replacement with the mobile style army which was clearly a stop-gap military force. The two emperors who reigned during the beginning and end of the previously mentioned time period, Diocletian and Theodosius, both fought to defend and keep the Roman Empire intact. Although the primary goal was the same, the respective armies that they used to work towards that goal were very different. As will be examined throughout this paper, the army of Diocletian greatly differed from that of Theodosius in four main groups: troop ethnicity, troop composition, weapons and armour, and battlefield strategy. Even though the Roman Army at its core between Diocletian and Theodosius was relatively strong and efficient, the problems alluded to in Diocletian's army eventually became reality in Theodosius' reign which led to the direct downfall of the Western Roman Empire.
The two emperors, despite ruling 90 years apart, Diocletian and Theodosius I faced the same problem, a military crisis. Diocletian came to power during the Crisis Period of the third century CE and immediately set out to stabilize the situation. The most prominent difficulty he faced concerned the army, which had grown out of control by the beginning of his reign. The army was divided into four main regions, and as a result when an emperor died, the four armies would often proclaim their own commander as the new emperor. As a consequence, the Empire would be plunged into another round of civil war, which only ended after one of the commanders had become sole emperor, and thousands of Roman soldiers had died. Diocletian set about repairing this problem with a two-fold solution, the first part consisted of creating the prefectures and dioceses on top of the provinces as part of a three-tiered system of governing and rule. The second part was the dividing of the military command and troops amongst several duces and across multiple provinces in league with the new three-tier system of rule. This new system, in theory, prevented one general or governor from controlling too many troops at once, and potentially avoided the chances of an ambitious legatus and army marching on Rome and seizing control of the emperorship.
The first step in the Diocletian army reforms had been accomplished, but the emperor had many more to implement. Despite the entire Roman Army numbering in the hundreds of thousands on paper, it was overall weak and susceptible to foreign invasions. To solve this, Diocletian began a massive recruiting campaign in order to replace the losses that plagued Rome during the Crisis Period and to further increase the size of the army. At its peak, Diocletian’s army numbered 435, 266. The next step was to protect the frontiers of the Empire which had been all but forgotten by the civil wars of the third century. The Sasanian Empire of the Eranshar, began to focus on expanding its borders into the eastern provinces of Rome, and new Germanic confederations were applying continuous pressure on the Danube and Rhine frontiers. Also, native Briton tribes were launching more raids into the Roman territories of Britannia. With the newfound stability at the centre of the Empire, Diocletian was able to focus most of his soldiers along the frontiers to defend the Empire from its various enemies. This can be seen as the opposite of the reforms of Constantine barely a generation later where the Roman armies were deployed in the interior of the Empire while the frontiers were left scarcely defended. At this point in Diocletian's reign, the core of his armies were still the legions, although their numbers were severely depleted. The number of soldiers in the average legion under Trajan numbered between 5,000-6,000, while the legions under Diocletian had been reduced to 1,000-1,200 troops each. The legionary losses from the Crisis Period were the prime cause of Diocletian's recruitment campaign but with a recognizable difference than the previous instances of recruitment. Instead of replacing all 5-6,000 soldiers per every Principate legion, Diocletian increased the total number of legions to around 70, but in the process, decreased these new legion's size to between 1-1,200 soldiers. This decision was a part of Diocletian's plan to improve and extend the frontier defences while at the same time limiting the amount of troops under a single commander.
75 years later, in 380 CE, Theodosius I ruled the Empire as sole Augustus and faced an equally dire situation as that of Diocletian. While Diocletian’s greatest threat was from within the Empire, that of civil war, Theodosius’ was from the outside, from the various barbarian raiders along the Rhine and Danube, and the continuous attempts of the Sasanian Empire to reclaim the old satraps of the Achmaenids. Although the Empire still suffered from infighting during Theodosius’ reign, these incidents were minor and did not last very long. By the latter half of the 4th century, the Empire’s frontiers were being pushed to the limit by constant barbarian incursions which forced some emperors to admit some of the migrating tribes into the Empire to settle. When a tribe was allowed to settle, they had to either enlist a group of its people, militarily called foederati, into the Roman Army, or pay money towards the upkeep of the army. In rare cases, they did not have to give either money or troops to the army. This program sometimes failed, such as in 378 CE when a Roman army was destroyed by a collection of newly settled barbarian tribes which was led by the Visigoths. These barbarians revolted against the harsh conditions imposed on them by their Roman rulers, which was a perfect example of the Roman imperialistic view of the barbarians. This result was the Battle of Adrianople, in which Valens, the Eastern Roman emperor, was killed along with the majority of his army. This was the event that raised Theodosius to the position of eastern emperor and the inheritor of the Empire's many problems. In his immediate, and not well considered decision, Theodosius, instead of recruiting Roman citizens into the army, began a series of recruitment campaigns targeting primarily barbarians from both inside and outside the Empire, from both allied and enemy tribes. This decision was personified in 380 CE, when some of the Visigoths who destroyed the Roman army at Adrianople were used to push other Goths and barbarian peoples back across the frontier. This new system of recruitment furthered the decline of the Roman Army. This was so because even though barbarians were considered natural-born warriors and highly sought after for that fact, their morale was not as great as the Romans and their loyalty was to themselves and their tribe, and not to the Empire. From Theodosius' reign until the official Fall of the Western Roman Empire in 476 CE, more foederati were recruited into the army than Roman citizens. The decline in recruitment of the Roman citizenry was directly caused by the dire military situation faced by the Empire. Furthermore, the Roman citizenry lost all interest in joining the Army and either paid money or cut off a thumb or limb in order to avoid military service.
The recruitment of barbarians into the army was not introduced by Theodosius, but it was his primary alteration to the army, though he was using the comitatenses and limitanei army model. This new model of the Roman Army was created by Constantine I, which divided the Army into stationary and mobile troops, and ultimately led to the barbarian threats faced by Theodosius. Constantine saw that a mobile army could be more useful than the frontier forces so he retracted the majority of the Roman troops from the frontiers and placed them in the strongest cities close to the frontiers. The frontier troops, called the limitanei, were the descendants of the legions but became second-class troops under Constantine. The mobile army troops, the comitatenses, were not descended from the any particular segment of the legionary army model, they were a new type of fighting force all together. They were deployed to the frontier when the limitanei were overwhelmed and needed the primary fighting troops of the Empire. This new army model was arguably more efficient than the legionary model due to the fact that the legions guarded their own set position on the frontier and had to move from it to another location if there was a raid or a major campaign was announced which consumed a significant amount of time, adding to this was the lesser mobility of the legions compared to the highly mobile comitatenses. Regardless of the differences in fighting prowess, tactics, and arms and armour, the comitatenses army was better suited to the needs and demands of the Roman frontier.
Theodosius’ recruitment campaign bolstered the Roman Army to the largest size that it had ever been, estimated at 635,000 on paper. In comparing the sizes of the respective armies of Diocletian and Theodosius, their main difference was seen in each emperor's reliance on barbarians, or the foederati. Allowing barbarians into the army was not necessarily a problem, but Theodosius made the mistake of relying too heavily on barbarian troops, as well as recruiting the barbarian troops from former enemies and allowing those same barbarians along with other foederati to be commanded by one of their own. These troops were less loyal and they would more likely let an invading force into the Empire and become a part of it, rather than defend the frontier. To further the trouble of the Theodosian Army, the number of new recruits from Roman citizens decreased exponentially, even in former major recruiting centers. By 476 BCE, when all that remained of the Western Roman Empire was Italia in name, the majority of troops left fighting were in fact the foederati. By entrusting the defence of the Empire to the barbarian troops, Theodosius abandoned the traditional idea of relying on Roman citizens to fight in the army which was one of the primary causes for the fall of the Western Empire. However, the only position that Roman citizens were found in the army of Theodosius was in the cavalry. During the last hundred and fifty years of the United Roman Empire, the quality of cavalry grew due to the rise in use of this type of troop by Rome’s enemies. Knowing this, only the wealthy and prestigious Roman citizens could afford to be in the cavalry which left the rest of the citizens to either join the infantry, or avoid it entirely.
The third main difference between the respective armies of Diocletian and Theodosius was in their weapons and armour. The arms of the average legionary soldier had remained mostly intact between the reigns of Trajan and Diocletian because they were very effective against the enemies faced by the Romans at the time. During the Crisis Period, the Roman armies were frequently fighting themselves which left no time to improve or replace their weapons and armour. Obviously Diocletian was satisfied with his army’s equipment and made no attempt to further develop the existing weaponry. The Roman State paid for the armour, and if better armour was more expensive, then it would have created a greater strain on the economy. If the legionarys had to wear armour on their arms and legs, much like the hoplites or the later Medieval soldiers, the upgrade would either have been paid for by the State, or most likely the soldiers would have had to raise the money themselves, as was witnessed in the Roman armies of the 6-7th centuries, which would have led to discontent and possibly mutiny.
While the lorica segmenta armour remained popular amongst the Roman troops under Diocletian, their weapons experienced a slight but important change. The gladius, the sword that had won Rome their empire, was being replaced by the Roman cavalry sword, called the spatha, which was longer and delivered a stronger impact, as the primary sword of the Roman infantry. Since the segmented armour was near impervious to the gladius, then a longer and stronger sword could theoretically be more damaging to this armour type. This was one reason for the replacement of the gladius. With Romans fighting Romans and having no clear advantage over each other, except for possibly different strategies, a definite change in arms and armour was needed. The primary use of the gladius was for thrusting and stabbing, not slashing, which was the spatha’s primary use in battle. This can be seen as an early and major influence of barbarians on the Roman Army. The barbarians had been using longer swords effectively against the Romans since the fifth century BCE and so it was only a matter of time before barbarian arms made their way into the Roman armies. The Roman legionary's secondary weapon, the pilum, remained the same in the army of Diocletian except for new tips being developed. These new types of spear-tips arose to respond to be not only more effective against Rome’s enemies, but towards other Roman soldiers as well.
While the Army witnessed a dramatic change between Diocletian and Constantine, it experienced a gradual change between Constantine and Theodosius. The lorica segmenta armour declined in use, partly because of the changing nature of Rome’s enemies and partly due to more foederates being used by the Army, who did not need or were not given the segmented armour by the State. Also, the declining numbers of Roman citizens joining the Army meant that the State did not have to spend money on the expensive armour. Furthermore, the barbarians were content with simple chain-mail armour. As the shift in importance from infantry to cavalry happened gradually from Constantine on, Theodosius' cavalry troops were drastically different from those of Diocletian. Due to the cavalry having battlefield superiority, they were given the best armour available. Comparing the lorica segmenta armour and chain-mail armour together, the segmented armour was better made and had a higher defensive capability than the simple chain-mail and better suited for longer combat. Swords and arrows would usually hit the segmented armour and glance off, whereas swords and arrows would not necessarily go through the chain-mail, but the force and tips of the attacking weapons would be clearly felt by the wearer and cause a range of damage, both externally and internally. While chain-mail was highly valued in the Early Republican days, by Theodosius’ reign, it was still used but most likely highly despised by soldiers used to the segmented armour. The change from segmented to chain-mail armour was a significant one, but Theodosius’ army also differed from Diocletian’s in terms of weaponry as well. Diocletian’s main fighting force was still the legions, which, despite the change to a larger sword, still retained the same standard weapons as the legions under Trajan. Theodosius’ soldiers made their swords their secondary weapon and adopted the spear as their primary weapon. The main reason for this was the switch of battlefield supremacy from infantry to cavalry and the infantry becoming primarily defensive troops. The emperors and duces both realized that a unit of cavalry could charge into an infantry unit and wipe them out with the infantry using just their gladii and pila, regardless of the quality of troops. By changing the pilum to a spear and using the spatha as a close-quarter combat weapon, the infantry could now easily defend itself against a cavalry charge. This widespread change in weaponry was made easier by the recruitment of more barbarians into the Army, because their weapons were already the spear and sword.
The third element of the difference in weapons and armour of the Army between the reigns of Diocletian and Theodosius were their shields. As with the gladius and lorica segmenta armour, Diocletian’s troops still used the scutum shield, which was used in many situations and for many roles. Due to its square and concave shape, it was easy to use to defend against an oncoming enemy, either in single combat or an entire unit. If a cohort was being charged by cavalry, the troops would lock their shields and brace themselves for the impact. This tactic would not always be successful because even though legionarys were able to fend off light cavalry with ease, cataphracts and other heavy cavalry were built to smash through infantry lines, regardless if they were legionarys or not. This inability to properly defend against heavy cavalry directly led to the replacing of the pilum with spears during the fourth century from Constantine to Theodosius. In doing this, the Roman emperors created a more effective infantry force because while the spear was an improvement in defending against cavalry, the use of the scutum by the Roman infantry was replaced with the oval cavalry shields of the Early Empire. This alteration was due to two reasons: the rise of barbarians in the Army who were already using round or oval shields and accustomed to combat with this type of shield, and the decline in the use of lorica segmenta armour which was handed out as a single package with the gladius, pilum, and scutum.
The third area where Diocletian’s army differed from Theodosius’ army was in their tactics and strategies. When Diocletian became emperor and reformed and stabilized the army, it was still using the standard legionary battle formation. This, in its basic form, was comprised of a Roman legion, an auxillary legion, an ala of cavalry. and light infantry and artillery. This formation could increase in size depending on the immediate number of troops in the army and any conditions that they were facing. In any situation and any battle, there was always a 1:1:1:1 ratio in the Roman Army. The only element that changed in the standard legionary army was its' numbers, caused by major defeats at the hands of Rome’s enemies, or in civil wars against other Roman troops. In the legionary army, and in the previous iterations of Roman armies, the infantry played the dominant role in their battles with the cavalry and light troops supporting the infantry. This strategy continued in Diocletian’s army because the enemies, being effectively the same as in Trajan’s reign, could be fought and defeated in the same way as well. Perhaps the only improvement to the troops in Diocletian’s army was that the Roman cavalry improved in their abilities and thus became more efficient, mainly due to the rise or the Sasanian Empire in the Eranshar.
While Diocletian’s army retained the same legionary battle tactics as the Severan Empire, the troop and strategy changes that were altered by Constantine directly affected later Roman armies, especially that of Theodosius. With the Army now divided between comitatenses and limitanei, there were new strategies implemented that reflected the new roles of these new troops types. The limitanei, the unfortunate descendants of the legions, had become permanent frontier troops and held no other duty than to guard the borders of the Empire. If an enemy attacked, then the limitanei were to hold their positions, if possible, until the mobile army could arrive and deal with the enemy invasion. Another tactic, the one that was more than likely to be used, was if the enemy broke through the frontier, then the limitanei were to block their exit in order for the mobile army to engage and eliminate the invaders. The limitanei were for the most part not used in battle, but could be whenever a general required it due to a shortage of troops or a quick force for defence. In these situations, the situation was dire enough for a general to use limitanei and not be concerned with the calibre of troops under his command. The mobile army of Theodosius was still using the basic legionary battle strategy, but with obvious changes. For example, in the Battle of Strasbourg in 357 CE, Julian lined his troops up into two distinct lines with his entire cavalry division on his right wing, this was mainly due to the marshy ground on his left wing. Because of the significantly lower amount of troops in the Late Roman Army, but higher numbers of legions, an entire 4th century Roman legion could only take up a small segment in the battle line. This disadvantage forced Julian and other Late Roman emperors and duces to either shorten the battle line while making it deeper, or stretch out the battle line but make it thinner. If Julian stretched out his line at Strasbourg, he might have outflanked the Alemanni but his centre would have been hard-pressed and most likely would have broken before the Alemanni attack; also, this possible event would have allowed the Alemanni to break-up the Roman line and subsequently roll-up the left and right wings of the Roman infantry. The two-line tactic, or deepening the line in plain sense, did not always work however, as Valens deployed a three-line battle formation at Adrianople and it cost him both his army and his life.
The infantry were not the only Roman military unit to be reorganized. The Roman cavalry was slowly given more precedence in battle from Diocletian to Theodosius and ever onwards. As more foederates joined the army as infantry, paired with the decline in number of Roman citizens entering the army, Roman generals began to rely more on their cavalry to deliver the deciding blow in a battle and to gain victory. Although cavalry in Roman armies became more prominent, there was a visible difference between Eastern and Western Roman armies and their respective cavalry units. The Eastern armies had been fighting the Parthians and then the Sasanians, who were primarily cavalry powers, for so long that they incorporated more cavalry forces into the army to match the numbers and strategies of the Eranian kingdoms. In most battles against the Persians, their armies included mostly or only heavy cavalry and horse archers. In the Western armies, they also experienced a rise in the use and numbers of cavalry but they were still fighting barbarian armies, who were predominantly infantry forces. It was not until the barbarian invasions of the Goths, Vandals, and Huns, plain and steppe peoples who excelled in cavalry troops, in the late 4th century and throughout the 5th century, that the Western armies adopted the early Eastern army model of balance between the infantry and cavalry. The balance between the infantry and cavalry eventually shifted to the cavalry by the middle of the early fifth century for the Eastern Empire, whereas the infantry still outnumbered the cavalry in the Western Empire. This rise of the cavalry during the time period of the 4th century Roman Empire was the beginning of the dominance of cavalry that lasted for over a thousand years.
In comparing both the army of Diocletian and the army of Theodosius, one can see many different elements but one major problem that faced the Roman Empire as a whole. Arguably, beginning in the late second century CE, the Empire slowly became less and less able to deal with the small barbarian raids which led to a much larger trend that was never completely repaired. This trend handicapped emperors who sought to raise the Empire back to the days of the 1st and 2nd centuries, but were further troubled due to the constant competition between their duces who would make attempts to become emperor themselves if the opportunity arose, especially if the emperor was inept or short on popularity. The massive numbers of troops under the command of Diocletian and Theodosius were more than enough to secure the Empire but they were only paper numbers. The total number of real Roman soldiers was better reflected in the numbers of the mobile armies and not the limitanei. The differences between the Diocletian army and the Theodosian army did not make them any less powerful when properly led, trained, and supplied, but the differences did lead to irreparable problems, just like that of the entire Empire, that no amount of reform could repair.
 Jones, A.H.M. The Later Roman Empire, 284-602. Pg.373
 Cameron, Av. The Later Roman Empire. Pg.39
 Jones. The Later Roman Empire, 284-602. Pg. 679
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 Southern. The Roman Army. Pg. 248
 Jones. The Later Roman Empire, 2840692. Pg.59-60
 Amm. 31.4
 Cameron. The Later Roman Empire. Pg. 138
 Cameron. The Later Roman Army. Pg. 137
 Cameron. The Later Roman Empire. Pg. 137
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 Jones. The Later Roman Empire, 284-602. Pg. 214
 Theodosius' successor, Stilicho, faced a much worse version of this problem
 Illyricum, Western Spain, etc.
 Jones. The Decline of the Ancient World. pg. 217
 solid interconnected plates on the torso, and overlapping plates on the shoulders
 Goldsworthy. Roman Warfare, pg.43
 Southern. The Roman Army: A Social & Institutional History. Pg. 261
 Goldsworthy, A. Roman Warfare. Pg. 167
 Early and Middle Republican, Greek-influenced, and Etruscan-influenced
 Amm. XVI.12
 Amm. XXXI.12.
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Ammianus Marcellinus. Rerum Gestarum
Cameron, A. The Later Roman Empire
Goldsworthy, A. Roman Warfare.
Jones, A.H.M. The Decline of the Ancient World
Jones, A.H.M. The Later Roman Empire, 284-602
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Southern, P. The Roman Army: A Social and Economic Institution