5 – The Roman Military Machine Collapses, Along With the Western Empire
Rome reached its territorial peak sometime around 117 AD under the reign of Trajan, but the sheer size of the empire created challenges the Romans were unable to solve. For centuries, the Roman army focused mainly on offense with the goal being to conquer as much territory as possible. From the reign of Hadrian onwards, the onus switched to defense as the empire desperately tried to hold off the vast number of barbarians encroaching around the Danube and Euphrates.
The Romans had to establish permanent camps for this purpose, but by the middle of the third century, the army could no longer cope. The fine-tuned Roman legions became a thing of the past as cohorts were drafted in to plug the ever-increasing number of gaps in defenses. In 212 AD, Emperor Caracalla offered full Roman citizenship to all provinces and various emperors in the third century were so desperate for troops that they recruited whoever they could regardless of the source. At that time, the non-Roman members of the army were not large enough in number to trouble the legions.
During the Third Century Crisis, the power of the military came to the fore as a succession of ‘barracks’ emperors took the throne after gaining the support of an army. As a result, emperors were murdered in quick succession, and the empire was in dire straits. From the reign of Gallenius onwards (the 250s), the importance of the heavy infantry dwindled as light infantry and cavalry were preferred. In the late third century when the Crisis ended, Emperor Diocletian created a central reserve to protect the empire. While Octavian had created a huge permanent army, they were all stationed at the borders. If barbarians broke through, there were limited troop available to stop them.
Cavalry was of paramount importance in the third and fourth centuries because barbarians typically raided Roman territory instead of launching invasions. The problem with having mercenaries as soldiers came to the fore at this time. Many of the marauding barbarians had served in the Roman army and were aware of its tactics and power. The Romans tried to come up with new techniques to use cavalry to support its infantry, but they suffered a terrible defeat at the Battle of Adrianople in 378 AD. Emperor Valens’ eastern army was destroyed by powerful Gothic cavalry.
Theodosius succeeded Valens and realized that Rome could no longer rely on infantry in battle. He bribed German warlords, and by 384, he had around 40,000 horsemen in his army. The gap between the Roman military in the East and West was brutally exposed when Theodosius annihilated Magnus Maximus, the usurper, in 387. The Western army still used legionnaires, but they were no match for the mercenary horsemen in Theodosius’ army. Five years later, Theodosius once again used his horsemen to demolish an enemy.
The need for speed became apparent in the Roman infantry of the fifth century in the west and east. While the old heavy infantry survived, new units were lightly armed with maneuverability the key. An increasing number of archers were trained to combat charging cavalry. Eventually, the German federates took control of the West and Rome, and the Western Empire fell in 476. The East survived by hiring soldiers from Asia Minor to prevent German dominance of the army and it issued, even more, reforms to help the empire last almost 1,000 years longer than its Western counterpart.