The reign of the Emperor Septimius Severus marked a dramatic shift in the organization and structure of the Roman army.
By Pat Lowinger
Ramsay MacMullen is an Emeritus Professor of History at Yale University following his retirement in 1993. In 2001 he was given a lifetime achievement Award for Scholarly Distinction by the American Historical Association, and is often credited with being one of the greatest living historians regarding the Roman Empire. To date, he has credited with no less than 16 books which detailed various aspects of the Roman Empire, including Paganism in the Roman Empire, Corruption and the Decline of Rome, Roman Government’s Response to Crisis A.D. 235-337 and many others. MacMullen’s earliest work, and the topic of this essay, was first published in 1963- Soldier & Civilian: In The Later Roman Empire.
In this work, MacMullen discusses in detail the events, pressures and transitioning economy of the Roman Empire which led to the dramatic decline in the strength, flexibility and martial prowess of the Roman army during the 3rd through 5th centuries CE. Using MacMullen’s work as a guide, and examining his thesis, perhaps we can gain a better understanding of those transitions which ultimately led the decline of the Roman Military. MacMullen’s thesis is perhaps best stated by the use of his own words:
The Roman Army, being used for ends not strictly military, lost its professional edge in a process stretching over perhaps two centuries, first accelerated by Septimius Severus and never reversed thereafter. Party, as a result, but more because of the violence of the later Empire, civilians had to arm themselves for the own protection. Civilian turned soldier, soldier turned civilian, in a rapprochement to the middle ground of waste and confusion. By the process, each influenced the other, but one of influence, the militarization of civilians, was particularly significant, and did much to change society. Such in sum is what this book tries to prove. 
During the reign of Septimius Severus, a major transition was beginning to take hold in the Roman army. The army which had been one of conquest and expansion was relegated to the protection of its frontiers against incursion by hostile raiders. In particular, the borders of Germania, Northern Britannia, and Africa presented the most difficulty in the maintenance of static defenses. The army was transformed from a standing army, to an army comprised of two major classifications- comitatus or mobile field armies and the limitanei which served as garrison troops on the frontier(s). It was the duty of the limitanei to secure borders, repel raids, and alert the field armies in the event of invasion. 
Unlike the mobile field armies, the limitanei were designed to be sedentary, more self-sufficient and less dependent upon a constant supply of logistical material and support.  In both practice and proscribed by Roman law, much of the land surrounding the border forts were property of legion which guarded them. While some of these lands were cleared for purely defensive purposes a significant portion was allocated to the production of food and the raising of cattle. The continuous maintenance of numerous armies was a severe burden on the coffers of the Empire. By having the limitanei supplement their own dietary needs, the net effect was a reduction in the cost of maintaining part of the Empire’s army.
With the establishment of permanent border garrisons, a new migration occurred. A migration of Roman civilians who would provide goods and services to garrison troops, be it trade goods, entertainment (often in its baser forms) and veterans who had been awarded lands upon completion of their contracts of service in the army. Some of these civilians established homes and workshops near or within easy travel of garrison forts. As a result, camps began to form at specific locations associated with the garrison; sometimes these camps were seasonal, but over time some became a permanent fixture upon the landscape.
While the mobile army relied on a host of skilled professionals and craftsmen for various needs, under Septimius Severus the limitanei were expected to provide much of their own needs. Whereas before, the army had largely relied upon large facilities- known as fabricae, located across the Empire for the production of arms, armor and metal goods, the border garrisons were increasingly expected to supply their own needs. What we observe is a dramatic change in the composition of the typical Roman fort. “Under Septimius Severus, however, the military market was largely and suddenly cut off. Two enclosures arose, served by detachments of soldiers who slept in the barracks of the East Compound and worked in the West Compound.” 
In the beginning of the 3rd century CE, changes were occurring within the Roman Army. The limitanei, who formed an increasing component of the Roman military, were forced to assign and even increasing portion of their labor to their own maintenance- to the noticeable detriment of the martial abilities. By the beginning of the 4th century CE, the limitanei, who had initially only supplemented their agricultural needs had been turned into a force or farmers and craftsmen.
Another reform by Septimius Severus was repealing the long standing prohibition (from the days of the Later Roman Republic) of marriage by soldiers. This change perhaps was in hopes to raise the morale of the army, and perhaps it initially did, but over time the ramifications were all too clear:
Official recognition of the fact that legionaries were human, however, started a chain of developments of which one sign can be seen in the abandonment of some of the barrack at York and Caerleon, where the men move out…Parts of forts were now given over to soldiers and their wives who chose to live ‘on the job,’ attested by the structural changes and by the ‘comparatively frequent occurrence with the barrack rooms of fragments of bracelets made of glass, paste, and jet, and of beads and similar trinkets. 
What occurred next was gradual, but marked a transition from a professional and highly mobile army into what could commonly be considered a militia. As urban centers developed near, and sometimes within the confines of the fort, the previous prohibition against the army living ‘inside’ a city was removed or ignored. In particular, the comitatus, or mobile field army was removed from the field and garrisoned within the larger cities which could support them. Unlike the limitanei who doubled as craftsmen and farmers, the comitiatus found itself tasked with guard duties, tax collection, local administration, and private trade- that is the ownership and operation of a business, often a family affair, in addition to the normal duties required of a soldier.
As an ever increasing result of these changes within the Roman army, veterans, particularly, those who were married settled in or very near to where they had been assigned as garrison troops. Rather than returning to Rome or Italy, where open public lands were scarce or inadequate, Roman veterani often retired to live in their ‘own’ communities or veteran’s quarters. In return, many of these Roman citizens of means turned to local politics- in fact many administration offices could only be occupied by former members of the military.  Thus there became a cycle of military service followed by civil office in an almost inseparable delineation of the two.
The implementation of multiple roles, reduction of discipline and removal of marriage restrictions were not without their own consequences- in the effectiveness of the Roman military:
A comparison of Rome’s military effectiveness at the start and at the end of the Empire is instructive, and depressing. In a battle of five thousand legionaries against as many enemies, a spectator in Augustus’ day would have given heavy odds on the Romans. Man for man, in physical strength and courage, Romans may have been no better than, say, the Gauls (whom Caesar eagerly enrolled)…The same spectator however, four centuries later, would have found it a very even struggle…A Roman victory had become- such is the impression one has, from accounts of the time- a mere fifty-fifty proposition. 
From profession full-time soldiers to part-time member of a militia the changes to the military in the Later Roman Empire may seem obviously detrimental to modern scholars. But these changes were not born out of absence of necessity. The Roman economy was diminishing, its internal politics often in shambles, and the state of its prominence in question. Continued external attacks certainly weakened the Empire, but this was secondary to the systematic decline of discipline and professionalism within the military.
Bibliography and Citations
 MacMullen, Ramsay. Soldier and Civilian in the Later Roman Empire. Cambridge: Harvard University Press, 1963. p. 152
 ibid., p. 6-7
 ibid., p. 15-16
 ibid., p. 25
 ibid., p. 126-127
 ibid., p. 110-112
 ibid., p. 152-153