How did The Roman Empire Start to Collapse?

The Roman Legacy - The Strength of Rome Endures.
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Mar 31, 2025
♦The Roman Empire didn't fall overnight. The Empire started unraveling long before the final collapse. This video breaks down the key events from 337 to 379 AD, right after Constantine's death , when things began to shift. Civil wars, power struggles , and pressure from outside forces started taking a serious toll. From the rise of new emperors to the growing threats along the borders, we cover how internal chaos and external enemies set the stage for the empire’s decline...
The Empire is starting to collapse.
The hard-fought stability brought to the Empire by Diocletian and Constantine the Great would
not prove durable.
While ensuring he had a viable dynasty to continue ruling following his death, Constantine's
son would not operate with the same tact and strategy as their father.
Beginning with the purging of perceived internal threats including close family, it wouldn't
be long until they turned on each other, bringing full-scale civil war back to Rome.
Alongside the internal strife, the period would be characterized by continued European
tribal threats which were becoming difficult for the imperial machine to sustain battle
against as well as a continuation of the radical move towards Christianity as state religion
begun under Constantine.
Constantine the Great's long reign gave way to the co-rule of his three sons, with
him investing his hopes for stability in a direct line of descendants sharing power.
Given the length and impact of his reign, his sons were afforded legitimacy and there
was no effective challenge to their assumption of rule.
The empire they inherited was a newly Christian one, still undergoing transformative changes
from its pagan traditions to embrace the new religion.
Threats on the borders also remained persistent, as they had for the past century or more now.
Tribal invaders constantly pushing west from northern Germany down to the Adriatic, and
the Sassanids a growing menace once again.
The period began with a bloody purge, instigated by Constantius, who ordered the killing of
many male relatives who could pose a challenge to the son's power, two uncles and several
cousins in total.
However, despite the attempts to smooth their path by the removal of potential challengers,
the peace between the brothers wouldn't last long.
By 340 AD, Constans and Constantine were battling in a civil war over Rome's western provinces,
while Constantius governed over the east, where he was consumed by a war with King Shapur
II of Persia.
Constantine II was the loser of this conflict, killed near Aquileia, Italy, as he attempted
to launch an invasion of the mainland to take territory from his brother.
This would leave just Constans and Constantius, each governing half of the empire with an
uneasy co-rule lasting through to 350 AD.
For much of this time, Constantius was preoccupied with Sassanid incursions.
Through the period, they would lay siege to a number of Roman cities in Mesopotamia with
their fearsome war elephants and siege machinery.
However, they remained unable to make key gains.
Meanwhile, in Europe, Constans continued making war on the Germanic tribes, something he had
been doing with success under the rule of his father, Constantine the Great, including
against the Franks and Alamanni.
There was division between the brothers in the 340s over their different interpretations
of Christian orthodoxy during this still early phase of the established and centralized church.
However, it wasn't until 350 AD that crisis would erupt again.
Of the two brothers, it was Constans who seemingly lacked popularity.
Salacious rumors of unknown veracity concerning his sexuality existed, as did his supposed
habit of ignoring wise counsel.
However, what is known is that a disgruntled general by the name of Magnentius attempted
to usurp rule and declare himself emperor in eastern France.
Shortly after, Constans was cornered by Magnentius' men in a fort in the Pyrenees and was killed
by them after fleeing to the temple of Helena there.
With the killing of Constans, Magnentius now tried to assert his legitimacy as emperor
of the Western Empire.
However, not from the Constantinian dynasty, it seems he struggled with this, and while
attempting some reforms, his three-year reign was defined by military encounters with Constantius
and his armies of the Eastern Empire.
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Constantius' eagerness to put down Magnentius was less to do with grief for his murdered
brother, and more to do with seizing the opportunity for sole rule of the empire and putting down
a threat.
Despite Magnentius' initial attempts to appease Constantius, including the minting
of him as superior on coinage, for example, the situation could not be allowed to go on.
Constantius marched his legions west, with the two sides meeting in a major clash at
the Battle of Mursa Major in modern-day Croatia.
This was likely the largest Roman-on-Roman battle in the history of the empire, with
as many as 60,000 men under Constantius and around 30,000 under Magnentius.
Details of the course of the battle are scarce, but while Constantius came away the victor,
losses on both sides were large, possibly as many as 48,000 casualties.
This wasn't just a blow in the short term, leaving the empire weaker against the growing
barbarian and Sassanid threats, but likely had repercussions on the rest of the history
of the Western Empire.
The replacement of that many men was a multi-generational project, which in this case, due to other
future pressures, would never be completed.
Following this defeat, Magnentius withdrew to northern Italy and towards his heartlands
in Gaul.
He was given a brief period of respite as Constantius refocused his attention on the
Sarmatian tribal threat along the Danube in 352.
But after achieving his aims, Constantius marched to meet Magnentius again.
This time, he retreated to Gaul, where the two men would meet for the final time at the
Battle of Mont Salucas in southeastern France.
This would be Magnentius' last stand.
His reputation amongst his men and territories under his control was already severely weakened
following the crushing defeat at Moussa Major.
Yet fate was not with him, and he suffered a defeat here again.
Thus ending his three-year insurrection, Constantius was now sole ruler and had finally consolidated
his rule.
He wasn't to bask long in this complete power though, and pragmatically appointed
a Caesar below himself as Augustus around 355.
This was a surprising decision for an emperor who had wiped out so many of his relatives,
but it perhaps reflected the pressures the empire was under.
The man he chose was Julian, who would serve in this role as ruler of the Western Empire
for the remainder of Constantius' reign.
Tied by blood, which was ideal, yet Julian had been immersed in academic pursuits in
Athens and was young, at 23 still.
However, he adapted to the new demands of the role and his region of Gaul.
Within a year, he was leading his first campaign with the task of pushing back Frankish marauders
in the area of the Rhine.
This he did with success.
However, he then decided to spend the winter resting his men and withdrew to modern-day
Verdun in France, dispersing troops around the area to try and solidify positions.
By doing this though, he left himself vulnerable.
The Franks, ever mobile and hungry for battle, seized on this opportunity, besieging Julian
and his reduced force size there.
Things nearly ended badly, until relief came for him some months later.
Not deterred by his close encounter at Verdun, the following year in 357 AD, Julian embarked
on a more significant campaign into the same territory.
This time, a planned joint operation between himself and Augustus Constantius, with them
pushing at the Germanic troops from different directions.
As circumstances would have it though, he would never join Constantius directly on this
campaign.
Forced to double back to defend Lyon from a surprise attack by the Laity tribe, before
being faced by King Cunodo Marius and a confederation of his Alemanni forces, the emerging battle
of Strasbourg between the two forces would be a decisive Roman victory, allowing for
the repair of the defensive line along the Rhine and subjugation of the Alemanni and
other groups for a period.
This was an epic meeting of the two forces.
Julian's army, in the absence of Constantius' forces, likely stood at around 15,000, with
the Alemanni coalition likely at around 20,000-25,000 strong.
The Alemanni king Cunodo Marius had been sure to summon a strong army for this encounter
against who he knew to be a capable leader.
The Romans had a cavalry portion of around 20% of total troop numbers, including both
lighter cavalry, ideal for harrying and routing, as well as horse archers and more heavily
armed cataphract-style units.
Both sides lined up to face each other, with the Alemanni having the advantage of dominating
a slight hill.
King Cunodo Marius had previously opted to remain detached from the battle, as Julian
was behind Roman lines so as to be able to command more effectively, though apparently
after a demand from the rank and file troops, he was obliged to lead the infantry lines
on foot.
This was an interesting reflection of differences between the sides and, ultimately, a reason
Rome would always have the advantage with its more formal military approach and emphasis
on sustained execution of drilled strategy.
It is thought the battle began with a harrying approach from Roman light-mounted archers,
carrying out a maneuver likely learned in previous decades in the Arab area of the empire
and conflicts with the Parthians.
They would approach Germanic lines close enough to fire off a volley before a rapid retreat.
These waves would inflict heavy casualties as well as lure enemy infantry to break formation
and pursue the archers.
However, the bait wasn't taken, so Julian sent in heavy cataphract cavalry.
These were elite units, well-versed in battling Germanic tribal groups.
However, what they didn't know this time was that King Cunodo Marius had cunningly
So when the cataphract attack came, they could leap out from among their own cavalry, attacking
the Roman units at close range and from below and amongst the melee, thus taking them down.
This maneuver was pulled off with skill, sending the Roman cataphract units into a rout, a
huge morale boost for the Alemanni troops to see these elite and feared Roman units
fleeing from the battlefield.
However, next came a mistake.
The Germanic infantry, filled with enthusiasm, launched a headlong frontal assault into the
tight Roman infantry formation.
The interlocking wall of shields held strong against the Alemanni waves, the Germans unable
to break the wall of thrusting spear and gladius.
Despite eventually punching a hole through the Roman center, they were driven back by
By this time, exhausted from battle, as well as increasingly hemmed in an encircling Roman
infantry, the Alemanni attempted to flee.
It is thought that many thousands drowned trying to swim across the Rhine, with another
6,000 killed on the field of battle.
These were comprehensive losses and would be enough to restore Roman dominance in the
area for a period, as well as the previous defensive line which had fallen apart in
previous decades.
Despite these years of successful campaigns together and their blood ties, things would
turn sour in the power-sharing agreement, this time while juggling threats at Rome's
border between them.
With Emperor Constantius faced with a renewed Sassanid threat in the east, he demanded half
of Julian's troops from Gaul.
This is a tall order for any commander, and it is thought that the insurrection originally
began with the troops rather than Julian himself.
However, he soon joined the insurrection and effectively mounted a challenge to Constantius'
rule.
By 361, Constantius moved to face him.
However, he would not survive the journey, coming down with a bad fever and dying in
early November.
He apparently named Julian as his successor, which seems a strange move given the events
at the time of his death.
It may be a tweak of the historical record by his successor, or a strategic move on the
part of Constantius to choose the least worst option.
As whilst Julian had essentially challenged his rule, he was still family and would at
Among other aspects, Julian was a notable philosopher, having been liberally educated
in the east of the empire for a long period in relation to other emperors of his time.
He was also a follower of the empire's pagan traditions and keen to return Rome to its
pre-Christian era.
This is why he would come to be labelled in a somewhat derogatory way as Julian the Apostate.
He had also proven himself in Rome's European provinces against the constant Germanic tribal
threat.
This was in both military and economic respects, as after succeeding in battle, he successfully
restored the war-ravaged provinces to prosperity.
His short reign, however, would come to an end in 363 AD, as he was killed while on campaign
against the relentless Sassanids in the east.
They now had the initiative in this theatre, and before his death, realising his army was
low on provisions, had ordered the retreat of his troops towards Samara and Roman territory.
However, his men were constantly harried by the Sassanids during their retreat, and it
was during one of these encounters when Julian rode back to check on his last units facing
Sassanid attack that he was mortally wounded.
Reportedly a spear, which penetrated his lower intestine and liver, as he had neglected to
wear his armour in favour of speed.
Jovian, his successor and part of Julian's elite bodyguard, came to power after a declaration
by the troops.
It was an inauspicious start to a doomed reign, though, as he was forced to resume Julian's
retreat from the Sassanids on the day following his appointment.
However, his army got stuck trying to cross the Tigris, and was forced to agree to humiliating
terms with the Sassanids just to allow his exhausted and undersupplied men to retreat.
They were granted many concessions, and alongside his retreat, damaged his legitimacy.
The remainder of his rule consisted almost solely of his march back towards Constantinople.
He would never reach it, though, dying by what is thought was suffocation by assassin
or accident in his tent.
Following Jovian's death, Valentinian was chosen by a council from a selection of candidates
and became emperor in February 364 AD.
He quickly appointed his brother, Valens, as co-Augustus to rule over the east.
Like Constantius before him, possibly a recognition of the wide array of external challenges facing
Rome which were more than the capabilities of one ruler.
Indeed, the issues facing Rome by this point were extensive.
Almost constant war on the borders with the Sassanids and Germanic tribes had placed great
strain on the economy, leading emperors such as Constantius to repeatedly devalue the currency.
Taxation was also raised to try and address economic shortfalls which put strain on the
peasantry, further sowing seeds of discontent.
Not to mention the brutal civil war waged under Constantius which not only decimated
the standing army, but also had its own grave economic costs, as well as loss of key European
provinces to Germanic troops which Constantius and Julian fought so hard to reclaim.
Valentinian's reign was dominated by constant military struggles in Europe against the Saxons,
Alamanni, and others, as well as trouble in Rome's British possessions with a hostile
alliance between the Picts and Scots.
By this time, a picture emerges of Rome's European empire severely buckling under the
pressure from tribal incursions.
Valentinian would die while on campaign against these European tribal enemies.
Apparently, while holding a meeting with a group of Germanic tribal envoys, he suffered
a fatal stroke while losing his temper with the group.
Aside from this military strife, Valentinian made sure to restore the path of Christianization
under Constantine the Great, which it deviated somewhat under Emperor Julian and his short
pagan restoration period.
Along with his brother Valens, they also ensured the removal of any succession threat from
their predecessor Jovian by the binding of his son Varonianus, likely by burning or complete
removal of an eyeball.
This illustrates the quite depraved nature of successionary conflict by this point in
Rome's history.
The seeking of power had almost no limits, and the more balanced days of the Republic
seemed like a distant dream.
Following Valentinian's death, his brother would continue to rule for a further three
years alongside Valentinian's son, Gratian, who assumed his father's responsibilities
in the Western Empire.
Their co-rule would give way to Theodosius I in 379 AD, who would be the last Roman emperor
to have sole rule over both eastern and western portions.
