Captive barbarians implore the mercy of Emperor Marcus Aurelius. An intellectual, who corresponded with the great philosophical schools in Athens and wrote his own Meditations, Marcus Aurelius spent much of his reign campaigning against the Germanic tribes beyond the Danube. He planned to add extensive new provinces in the area of Bohemia, but the project was abandoned on his death.
The middle of the third century witnessed frequent Roman defeats. Germanic tribesmen raided deep into the western provinces of the Empire, while most of the east was overrun by the Sassanid Persians. In 251 the emperor Decius was killed when his army was defeated by the Goths. In 260 the emperor Valerian and his army surrendered to the Persians. A short-lived independent empire in Gaul and another in the East based around the kingdom of Palmyra threatened to fragment the Empire. Endemic civil wars sapped the army’s strength and weakened its capacity to fight foreign wars. Some stability was achieved under strong emperors such as Aurelian, Diocletian and Constantine, but only after the army’s structure and deployment had been greatly altered. Nevertheless, by the early fifth century most of the factors which would lead to the collapse of the western empire were already in place. The reasons for Rome’s fall are both complex and fiercely debated, and a full discussion of these would be out of place here, but it is important to consider whether military weakness and inefficiency played a major role in this process. Did the Roman army evolve to deal more effectively with a changed situation, or was it simply in decline?
Diocletian (Gains Aurelius Valerius Diocletianus) was a strong emperor who campaigned successfully against the Persians and usurpers in Egypt and Britain. One of the Illyrian emperors, he created the Tetrarchic system in which he shared rule with a colleague, Maximian, and each had a deputy. Diocletian abdicated in 305. During his reign many army units were withdrawn from the frontiers and stationed inside the provinces, the origins of the later comitatenses, and the army seems to have markedly increased in size.
It is tempting to view civil conflicts as aberrations, campaigns which distracted the army from its primary role of defending the Empire, but which did not have a profound effect on its structure and deployment. However, civil war was a common occurrence in the later Empire and accounted for the greater number of pitched battles fought by Roman soldiers. The shape of the fourth-century army grew out of long years of internal conflict.
The emperors of the early Principate took great care to maintain the loyalty of the army and were largely successful in this aim. There was nearly a century between Actium and the outbreak of widespread conflict at the suicide of Nero in ad 68, and an even longer period before the great wars following the assassination of Commodus in 193. There were attempted rebellions by provincial governors in between these massive struggles, but none gained much momentum. Some emperors met violent ends, but their deaths and the succession were usually decided in Rome. The Praetorian Guard had long played an active role in politics, notably proclaiming Claudius emperor following the murder of Caligula and auctioning the Empire to the highest bidder after the assassination of Pertinax. No emperor could survive without a loyal guard and most took care to treat the Praetorians well: they enjoyed far better conditions than the legionaries, serving for only sixteen years, receiving higher pay and frequent lavish donatives. Emperors like Galba and Pertinax, who failed to satisfy the guardsmen’s expectations with suitable rewards, met swift ends. Since no emperor wished to give command of the only troops in Rome to a potential rival, the Praetorians were commanded not by senatorial officers but by equestrians. Power and the succession remained focused on Rome, but this began to change slowly as emperors such as Trajan and Hadrian spent long periods in the provinces. Marcus Aurelius fought for years against the tribes on the Danube, dying on campaign in 180. Septimius Severus, who had fought a long civil war and then led an army against the Parthians, died in 211 at York (Eboracum) where he was wintering during a campaign against the Caledonians. More than twenty emperors held power briefly until the accession of Diocletian in 284, and there were many more usurpers who failed to establish themselves and died in the process. Civil wars were common and the majority of emperors died violent deaths, frequently at the hands of their own soldiers.
Shapur I was the son and successor to the founder of the Sassanid dynasty, Ardashir I. He campaigned with great success against the Romans, defeating and killing the emperor Gordian III in 244 and capturing Dura Europus in 255. In this relief from Nagshe Rostam, another emperor, Valerian, is shown cowering in front of the king after his defeat and capture at Edessa in 260. One lurid tradition claimed that after his execution, Valerian's body was stuffed and put on display.
As emperors spent more time with the armies in the provinces, so these began to behave in a similar way to the guard. The situation was worsened by changing patterns in the appointment of the senior officers of the army. The tradition began to change whereby Rome’s armies were commanded by senators who interspersed periods of military service with civil appointments. The presence of the emperor with the army encouraged especially lavish rewards for the men who distinguished themselves in his campaigns. An increasing number of equestrian officers were elevated to the Senate and went on to hold even greater military responsibility. Marcus Aurelius promoted a prefect of an ala, Marcus Valerius Maximus, giving him senatorial rank and making him legate of six legions in succession, because he had killed with his own hand a king of a Germanic tribe, the Naristae. The future emperor Pertinax is said to have failed to gain a commission as a legionary centurion, but, having accepted the prefecture of an auxiliary cohort instead, served with such distinction in equestrian posts that he became a senator. Severus raised three new legions (I—III Parthica), but entrusted their command to equestrian prefects rather than senatorial legates. Through the third century the number of senior army positions held by senators gradually declined. Far more opportunities lay open to equestrians, especially those who campaigned under the emperor himself.
The equestrian officers who dominated the third- century army were in many respects professional soldiers, owing their advancement purely to their military record and the favour of the emperor. The successful men were career soldiers who served with the army continuously and did not hold civil offices. It was usually these men and not the ordinary soldiers who plotted to murder an emperor and nominated a usurper from their group. It is possible, but by no means certain, that the rapid promotion and great responsibilities given to equestrians who distinguished themselves increased the quality of the senior officers of the Roman army. The distancing of prominent senators from the command of armies may at first have been intended to prevent potential rivals from gaining support in the army. In the long term it had the effect of making usurpations more common. Long service with the army allowed this new breed of professional officers to develop strong bonds with the junior officers and soldiers. A senator hoping to gain the Imperial throne needed to be confident of at least a fair degree of support and acceptance within the Senate if he was to stand much chance of a successful reign even after he had achieved military success. In practice this reduced the number of potential rivals to a relatively small group, primarily those able to achieve command of one of the large military provinces. The emperor’s frequent absence from Rome distanced him from the political world of Senate and capital, moving the court to the army. Since popularity with the soldiers and especially their officers was ail that was necessary to reach the throne, it became far easier for usurpers to mount a successful challenge. Once they had attained power then this depended on the continued support of their own forces and on having greater military strength than any other rival. Several of the most successful emperors of the second half of the century came from a small group of ‘Illyrian’ officers, a number of whom proved very able. Internal instability had led to losses and defeats on all frontiers of the Empire and further encouraged internal rebellions. Each emperor was required to campaign with little break, since he could rarely afford to entrust the command of a major army to a potential rival. When the emperor chose to operate for any length of time in one theatre of operations there was a great danger that other parts of the Empire, feeling that their own difficulties were being neglected, would create a rival.
Lucius Septimius Severus (ad 145-211) was the eventual winner of a series of civil wars, defeating his last rival, Clodius Albinus, the governor of Britain, in 197. He mounted a highly successful Parthian expedition, and spent the last three years of his life in a major war against the Caledonian tribes in northern Britain, dying of natural causes in York. Severus granted Roman soldiers the right to make legal recognized marriages.
Diocletian is often credited with strengthening the Empire after the chaos of the mid third century, hut in fact the Tetrarchic system only really functioned effectively during his reign. Later, it simply provided a framework for advancement amongst those aspiring to imperial power. Civil tears started to break out within a short time of his retirement, and his own fellow Augustus, Maximian, who had also abdicated, tried to resume power to aid his son.
A measure of stability was created by one of the ‘Illyrian emperors’, Diocletian, who gradually developed a system of dividing the imperial power, known as the Tetrarchy. This was not entirely without precedent: for instance Marcus Aurelius had ruled jointly with Lucius Verus until the latter’s death. In its evolved form there were two senior emperors, each known as Augustus, who ruled the eastern and western provinces respectively, assisted by a junior colleague or Caesar. Diocletian became Augustus in the east and chose another Illyrian officer, Maximian, as his colleague in the west. The system was designed to provide enough commanders to deal with several crises simultaneously, but, by nominating the Caesares as successors to their senior colleagues, to prevent civil war by providing for the ambitions of all men with armies. However, the system collapsed almost as soon as Diocletian and Maximian went into voluntary retirement in 305, in the main because it failed to make provision for members of the Imperial family who were not appointed to the Tetrarchy. The principle of divided power did last for much of the fourth century, apart from the period from 324—37 when Constantine managed to achieve sole power, but usurpations and civil war continued to be common. It is frequently forgotten that Constantine, the most successful emperor of the period, spent half of his reign as a usurper. The different grades of Caesar and Augustus and the acknowledgement of the east-west division allowed ambitious generals to progress in stages to ultimate power, but also left greater potential for compromise between rivals.
The strategy in civil wars was invariably simple. Unless a compromise was reached, such as the recognition of the rival as a ‘junior’ Caesar or as Augustus in the other half of the Empire, then the conflict only ended with the death of one of the rivals. Both sides gathered as large an army as they could and sought out a battlefield encounter. Such a clash was only delayed as each side sought to fight the battle in the most favourable circumstances or attempted to persuade the rival army to defect. It is only in civil wars that we hear of the cautious jockeying for position before a battle which was such a feature of earlier warfare. Fighting a similarly trained and equipped enemy denied the Romans the advantages they normally enjoyed in wars against foreign opponents, and placed great emphasis on mobilizing greater numbers of troops. Civil wars drew a high proportion of the army’s strength away from the defence and control of the provinces.
Constantine the Great (ad 285-337), the first emperor to embrace Christianity, teas proclaimed emperor by the army when Constantins, his father, died in 306. Defeating his rivals Maxentius and Licinius was his greatest military achievement, hut only in 324 did the latter’s death leave him as sole emperor. He completed the re-organization of the army into comitatenses and limitanei.