After the civil war that transformed Rome from a republic to an empire, emperor Augustus reduced the number of legions from 60 to 27,
which fell further to 24 and then rose back to 35 in later decades.
Recruitment relied increasingly on non-Italian peoples.
The traditional distinction between Romans and auxiliaries blurred, especially after the Antonine degree of 212 CE.
Still the legions provided the heavy infantry and the auxiliaries the bulk of cavalry and light infantry.
All legions employed substantial numbers of engineers who could build fortifications, conduct sieges and construct and operate artillery, but also engage in civil building projects.
The army became more and more garrison-like, bound to their bases by ties of race, language and culture, though kept largely separate from the civilian population.
By the late 3rd century CE, most of the army was made up of these garrison forces, who were named "limitanei".
They were supplemented by smaller numbers of "comitatenses" (companies), "palatini" and "scolae", palace troops who were attached to the emperor personally.
These were used not only to defend the empire against attack from outside, but also from the inside: upstart would-be emperors.
The limitanei made up roughly 75% of the army; the other types combined the remaining 25%.
The size of the legions was drastically reduced, leading to regiments ranging from 400 to 1,200 men and other types of units.
Yet the army remained a professional force, trained constantly, equipped by the state and paid by taxes.
The main task of the late Roman army was to defend the empire from barbarians, especially from the north and northeast.
It also had to fight many fruitless wars against the Sassanid Persians in the east.
The borders were guarded by a string of fortresses, fortified garrison towns and supply dumps, all tied together by good roads and rivers.
In Britain the Romans even used border walls: Hadrian's Wall and the Antonine Wall.
These static defenses were combined with patrols, punitive expeditions and pre-emptive strikes, but also barbarian buffer states, supported by Roman pay.
After the weakening of the army in the 3rd century CE, the static defenses became more important.
In the 1st century CE the army adopted banded armor, the lorica segmentata, which is the type of armor that Roman soldiers are usually portrayed with in modern times.
However scale and mail armor also remained in use
and in the 3rd century CE the lorica segmentata lost its popularity again.
Scuta changed from square to oval shape and helmets were strengthened.
Together with the body armor, these kept the legionaries well protected.
The gladius was replaced by the longer spatha;
pila were replaced by lighter javelins and "plumbata", throwing arrows;
stabbing spears re-appeared;
archers were equipped with composite bows and crossbows.
The Romans copied heavy cataphract cavalry from the Sassanid Persians.
In the 4th century CE they also created even heavier armored cavalry named "clibanarii" (camp oven cavalry), named so because the armor that they wore was very hot.
In the east, the army relied more on archers, both infantry and cavalry.
At sea, with the Mediterranean basically a Roman lake, there was no more need of large war fleets.
Instead the Roman navy employed small squadrons of small ships to police both seas and rivers and combat pirates.
Despite the lamentations of historians and its eventual failure, the late Roman army was an effective fighting force that defended the borders of the empire successfully for several centuries.
It was the erosion of the civil tax base that made its quality decline from the late 3rd century CE onward.
Soldiers got less pay, volunteers became supplemented with conscripts and desertion became a problem.
More and more use was made of "foederati", foreign troops who where led not by Romans, but by their own leaders.
Eventually the army could no longer stop the barbarians from the north, who destroyed the western Roman empire, taking a full century to do so.
In the east, Rome and its armies prevailed as the Byzantine empire for a full millennium.
War Matrix - Late Roman army
Roman Decline 120 CE - 480 CE, Armies and troops