After the military reforms of Emperor Costantine the Great (306-337 A.D.), the traditional Roman army as we know it morphed into a more mobile and specialized force, able to deal quickly with foreign and domestic threats that arose throughout the empire. The new army of Costantine was composed of three main groups: the Comitatenses, the Limitanei or Riparienses and the Palatina legions.
The Comitatenses, heirs of the classical roman legionnaire, were the professional heavy soldiers armed with the long spatha and an oval shield. Their commanders were the Comes, a military figure who later gave birth to the medieval Count.
The Limitanei or Riparienses were militia who guarded the precarious roman borders, operating as an initial defensive line and an efficient scouting force. They could be withdrawn if the situation got extremely dangerous. Their commanders were the Duces, a word later used to indicate all kind of commands.
The Palatina legions, which included the Ioviani, were elite regiments of soldiers and guards associated with the emperor’s presence. Contrary to popular belief, these legions were extremely successful against enemy incursions because of their structure but were considerably drained by civil conflicts.
This new defensive model provided by the army of Costantine was called “in depth”, as it guaranteed different levels of protection.
As can be seen in the Battle of Strasbourg (357 A.D.), the late Romans, guided by Caesar Julian, were more than capable of forming disciplined and effective armies. Therefore, we cannot solely blame the army for the Fall of the Western Empire. In fact, the army remained a capable and efficient force almost until the last breath of the Roman West.
To further prove this, we can look at the Battle of Adrianople (378 A.D.), during which there were no morale or disciplinary failures by the army, with troops retreating only when the situation became hopeless. The late Roman army was not weaker than its predecessor, instead it had to face extremely precarious situations, often prevailing over insurmountable odds.