How Did the Army of the Roman Empire Evolve?

This article is an edited transcript from Roman Legionaries with Simon Elliott, available on History Hit TV.

Dan has his regular catch-up with Simon Elliott on all things Roman. Why were the legionaries so successful, and how did they maintain that success for several centuries?

Listen Now

For centuries, the army of the Romans dominated the Mediterranean and we remember it today as one of the most effective forces the world has ever seen.

Yet to ensure the Roman army was able to compete against various enemies – from the swift Parthians in the east to the menacing Celts in northern Britain – evolution was necessary.

So how did this army change tactically and operationally from Augustus onwards? Was there any rapid development in battlefield technology and tactics? Or was there a cradle of continuity?

Continuity

If you look at the legionaries from the end of the reign of Augustus (14 AD) through to the legionaries at the beginning of the reign of Septimius Severus (193 AD), there was not a huge amount of change. The Roman soldiers we grow up reading books about, wearing lorica segmentata and having the scutum shields, pila, the gladius and the pugio, did not dramatically change in that time period. The military formations did not really change in that time period either.

You therefore tend to start looking at the evolution of Roman military tactics and technology from the time of the emperor Septimius Severus, and if you look at some of the arches and monuments in Rome – for instance the arch of Septimius Severus – you can still see there on that arch the Roman auxiliaries and their lorica hamata chainmail and the legionaries in segmentata.

Similarly on the Arch of Constantine, created towards the end of the fourth century, then you’re looking again at the changing technology. But even there on this much later arch you still get legionaries wearing lorica segmentata. Still, if you want a clear pathway of this change of technology and tactics you can see it beginning with Septimius Severus.

The Severan reforms

When Severus became the emperor in the Year of the Five Emperors in AD 193 he immediately began his military reforms. The first thing he did was abolish the Praetorian Guard as it had functioned so poorly in the recent past (even contributing to the demise of some of the emperors who did not last very long during the Year of Five Emperors).

The Praetorian Guard proclaim Claudius emperor.

So he abolished it and he replaced it with a new Praetorian Guard which he formed from his own veteran soldiers from the legions he’d commanded when governor on the Danube.

Suddenly the Praetorian Guard transformed from being a fighting force based in Rome, to one composed of elite soldiers. This provided the emperor a core body of men in Rome, and let’s remember throughout the Principate the legions tended to be based around the borders not within the Roman Empire. It was therefore very unusual to actually have a proper military force in Rome itself.

Alongside creating the fighting Praetorian Guard, Severus created three legions, one, two, and three Parthica. He based Legio II Parthica just 30 kilometres from Rome which was a clear message to the political elites in Rome to behave or else as this was the first time a full, fat legion had actually been based in close proximity to the heart of the empire.

The reformed Praetorian Guard and his new legions therefore provided Severus two big units around which he could build a mobile army if he wished. When Severus then increased the size of the horse guards in Rome, he then had what is effectively this embryonic mobile army which was the core of the force which he took with him when he campaigned to try and conquer Scotland in AD 209 and 210 before he died in York in AD 211.

Dan talks to Simon Elliott about Septimius Severus, about his Northern Campaigns and the true story of this savage 3rd century invasion of Scotland.

Listen Now

Later transition

Severus was the beginning of the change. You can then run through to the time of Diocletian when there occurred a transition to having mobile units within the empire and fewer smaller units along the borders. By the time you get to Constantine, you have a full transition where the core of the Roman military was not the classic division of legionaries and Auxilia but was much more focused on these mobile armies – including larger cavalry contingencies based deep within the empire.

Ultimately you had this split between Comitatenses, the field army troops, and Limitanei, which were effectively gendarmerie who were along the borders acting as a trigger for any penetrations into the empire.

So there was a clear arc of change in developments, in tactics, in technology in the Roman army, but it did not begin until around the time of Septimius Severus. For the majority of the Roman Imperial Period the iconic Roman legionary, equipped with their lorica segmentata and scutum shields, remained a constant.