The most interesting point in modern scholarship is there was no structure decline in the Roman Empire prior to the fall. Previous generations of historians simply assumed that the Roman military or political structure must have weakened or failed, but there is little evidence to support this hypothesis.

Over a century of time, from 376-476 AD, the Roman Empire suffered a series of exogenous shocks from foreign invasions which destroyed its economy and political system.

The fall of Rome demonstrates how vulnerable economic structures are to hostile attack.

The Romans had fought off excursions by the Persians, the Sarmatian Steppe nomads, and the Germans for generations. The sudden inability to do so in the 4th and 5th centuries demands an explanation.

The Romans established limes (frontiers) at the Rhine and Danube Rivers in Europe, along the Sahara Desert in Africa, and along the Syrian-Armenian border in the Middle East. The limes protected the best agricultural land and represented the economic limits of the Empire, rather than a constraint imposed by external foes. Interestingly, the Romans needed no walls for their interior cities or any other form of defense in depth. The Roman military provided a secure economic environment to facilitate sophisticated trade, manufacturing and agriculture. The Roman navy protected the sea trade and suppressed piracy.

Most of the Roman Legions were stationed along the frontiers, while allied client states ruled the territories past the frontiers. Client States raised their own military forces and fought off minor threats, reducing the threat to Roman territory. The Roman economy thrived behind this protective frontier.

The on and off trade with the neighboring Parthian Empire in Persia provided an economic connection with the rest of Eurasia, including China and India. The Romans were fond of silk clothing from China, for instance. With the silk, flowed a large quantity of cheaper goods. The Roman economy flourished from the safe and secure markets and economy of scale.

The Empire underwent a series of contemporary political reforms – namely the division of the Empire into Eastern and Western halves, each ruled by an Emperor and Caesar – which reduced the occurrence of civil wars. This evidence disproves the Decline Theory.

Peter Heather notes that there were structural changes outside the Roman Empire which were more significant than any supposed structural decline within the Empire. A series of exogenous shocks overwhelmed the Roman military and frontier which disrupted and ultimately destroyed the economy and polity.

The first exogenous shock was the fall of the decentralized and weak Parthian Kingdom in Iran and the rise of the significantly more powerful Sassanian Dynasty during the Second and Third Centuries. The old Parthian Kingdom was a confederacy, run by the northern Iranian pastoralists and herders. They were rarely unified and never posed an overwhelming threat to the Romans. The new Persian Empire under the Sassanians represented the urbanized southern Iranians. This Dynasty was centralized, efficient, and posed a more significant military threat. The Romans were rocked by several military defeats and in a military crisis they barely fought the Sassanids to a stalemate along the Syrian/Iraq border. The Roman Empire stationed a full 25% of its legions in the Eastern Empire along the limes in Egypt, Syria, and Anatolia. These troops could never be redeployed to Europe without encouraging a Persian invasion of Syria. Armenia remained a contested buffer state.

The second exogenous shock was a series of environmental and political changes in the Asian Steppe. The Steppe nomads grew more aggressive and expansionary. Skilled horse archers with composite bows descended on wealthier but politically fragmented agricultural states throughout Asia and Eastern Europe. This was the first of several mass migrations during this period. The Huns were one of the major nomadic groups that invaded Europe and their migration westward pushed the Goths and Germans into Roman territory. The Huns use of horse archers gave them a decisive military advantage on the Russian Steppe and in Eastern Europe down to the plains of Hungary. In 376, the Huns defeated the Goths in Eastern Europe.

Between 475-80 and 405-10, the Huns pushed the Gothic and Germani Tribes out of their homelands and into Roman territory. The Huns did not directly destroy the Roman Empire and Attila the Hun came considerably later after the damage was already done. The Huns started a mass migration in Europe.

The Germanic tribes underwent a form of structural revolution in response. Over time, the Germans became better at making iron weapons and tools through contact with the Romans, but they remained politically fragmented in warring tribes for centuries. The pressure from the East forced smaller tribes to form larger and more cohesive confederations than ever before. Goths fled the advance of the Huns and headed to the highlands and forests to minimize the threat of horse archers. These Goths ran into other German tribes and the limes of the Roman Empire. This set off a chain reaction as one migrating group pushed other groups off land. Eventually, by the mid to late 4th century, the Goths and Germans crossed the Rhine and Danube Rivers and the Roman imperial limes failed to contain the migrations.

The Roman military fought these invasions with mixed success. They were able to fend off some migrations but at the cost of allowing others through. Some groups defeated regional Roman legions and signed ceasefire agreements recognizing the Gothic conquest of land.

One Gothic group fled the Huns and crossed the Danube. The Eastern Roman Empire denied them access, but the Goths were armed and belligerent. They fought at the battle of Hadrianopolis in 376, resulting in a Gothic victory and the death of the Eastern Roman Emperor and much of his army. The Goths did not capitalize on their victory. The Roman legions reorganized and fought the Goths to a stalemate through guerrilla tactics.

The Huns continued pushing West and drove ever more Germanic groups to cross the Roman limes. By 401, Goths invaded Northern Italy. The Romans withdrew legions from Britain and the Rhine to defend their homeland. Soon afterwards, other Goths and Germans crossed the Rhine River en mass and overran much of Gaul, while Anglos and Saxons raided Britain. The reinforcements in Italy were not sufficient to stop the Gothic invasions there. By 410, the Goths sacked the capital city of Rome.

The Roman military system was simply overwhelmed at too many points in their defense. In the past, the Romans could withdraw troops from one end of the lime to reinforce another since German confederacies were rare and short-lived. Temporary movements of troops was feasible. During the 4th century, this was no longer the case. The limes were pressured at all points. A breach in one region would lead to a cascade failure allowing more breaches in other areas. Since many Roman cities were unwalled and their citizens unarmed, the barbarians swiftly conquered the interior and enslave the Romans civilians. The Roman economy was divided into segments and the tax base which supported the Roman legions collapsed. Many Roman Provinces were left to their own defenses, relying on untrained and under-equipped militias to fight off the war-like Germans.

The Huns, under the command of Attila, continued their rampage across Europe in the 450s, crossing the Rhine into Gaul and circling down to Northern Italy. Finally a Roman-Gothic Alliance defeated the Huns in Northern Italy in 452. The Huns actually overstretched themselves. They fatigued too many of their horses, causing many to die, and were unable to replace them with Europe’s limited number of horses and poor grazing land. The Hunnish Empire fell apart soon after the defeat.

The fall of the Huns ended the three-way balance of power between the Huns, the Germans, and the Romans. From 452 onwards, the Germans and Romans were locked in a death struggle. A Western and Eastern Roman expedition to retake North Africa failed, which marked the last Roman offensive before the fall.

The Germani Tribes took over most of the West. The Frankish Tribes overran Gaul. The Visigoths conquered much of Spain and the Vandals crossed to Africa and conquered Carthage. The Ostrogoths conquered Northern Italy and Rome by 476. The Eastern Roman Empire held out by ruling parts of Naples and Sicily, Greece, and the Middle East.

The loss of each territory was economically crippling. Mediterranean Sea trade halted. North Africa no longer fed Rome, the mines of Spain no longer supported manufacturers of Italy, and the specialized economy disappeared. The Western Roman Empire collapsed.

The Germanic Successor States established small fragmented kingdoms on the ruins of the Western Roman Empire. The Ostrogothic King of Italy, Theoderic, claimed he was the legitimate successor of the Roman Empire. This was mostly propaganda. The economic sophistication was gone. So were many Roman technologies, mathematical knowledge, and widespread literacy.

The Eastern Roman survived because of the Bosporus Straights. This waterway, defended by the Roman Navy, protected the Eastern Roman tax base and food supply in Egypt and Syria.

This was a highly violent transformation, not a peaceful cultural transition which later revisionists claimed. The end of the Roman political system led to the breakup of the Roman economy into tiny fragmented German fiefdoms. Dark Age feudalism was the result of this poverty.