Why did the Western Roman Empire collapse but not the Eastern Roman Empire?

At the end of the centuries long Pax Romana-(or "Roman Peace"; around the 200's AD /CE), Germanic tribes from Northern and Central Europe-(referred to as, "barbarians" by the Romans and Greeks........who invented the word), began to invade the Italian peninsula with numerous fighters. Germanic tribes, such as the Goths, Visigoths, Ostrogoths, Vandals, Teutons, Franks, Lombards, Angles and Saxons proliferated throughout Roman colonial Europe, primarily into Italy, as well as pouring into the Roman colonies of Northern and Western Europe.

The lands of Ancient Northern and Western Europe were primitive and largely undeveloped-(except for the well built Roman roads). There were few cities North and West of the Alps during the Roman Empire. London-(originally known as, "Londonium"), was a swampy marshland during Roman times, though towns, such as York in Northern England, as well as Trier, in Western Germany, were more sophisticated and developed-(as one can see from the preservation of its Roman archaeological sites). However, the majority of Northern and Western Europe during the Roman colonial age, was, (when compared with the Eastern half of the empire), poorer, less developed and culturally unsophisticated. This part of Europe were the badlands and wilderness of the Roman Empire-(with FEW notable exceptions).

The pastoral nature of Ancient Northern and Western Europe was a perfect spot for the Germanic tribes to settle into and conquer. Although there was the impressively built Hadrian's Wall in the North of England, it would do little to protect the Roman colonial West from Germanic militaristic onslaughts elsewhere. When compared with the Eastern half of the Roman Empire, the Western half was largely unprotected and unguarded, due to poor funding, as well as disregarding its overall geopolitical value to the Empire.

As for Italy proper, the North of Italy, during Roman times, was mostly, a primeval, forested wilderness-(with the notable exceptions of Verona, Assisi and Milan.....which became the Defacto Capital of the Roman Empire beginning with Constantine). The rich Latin speaking cities of Italy proper, were, Rome, Ostia, Tivoli and before its volcanic eruption, Pompeii, as well as nearby Herculaneum. The historically Greek cities of Southern Italy and Sicily, such as Taormina, Syracuse and Naples, were also rich Latin speaking cities during the heyday of the Roman Empire. If you were an ethnic Roman living during and after the Pax Romana, you would have much preferred Rome, as well as the cities and towns to its South, rather than the rural villages directly to its North.

But even the affluence of Central and Southern Italy during the Roman era, was waning when compared with the towns and cities to its East..........namely, the Greek East.

Emperor Constantine-(and even his predecessor, Diocletian), "saw the writing on the wall". Diocletian and especially, Constantine, recognized that the future of the Roman Empire was to the East of Rome. Although Milan would become the new administrative capital of the Western Roman Empire, it was the centuries old city of Byzantium that was of growing important strategic value.

The Germanic tribes, were primarily a landlocked peoples who had little or any familiarity with the sea; their invasions throughout continental Europe were led by primitive armies, though not by sophisticated navies. In other words, it was the strategic presence of waterways that Constantine and his generation recognized in Byzantium. The city was at the end of Europe, though directly across from Asia, as well as within reasonable sailing distance from Alexandria, Egypt-(the starting point of Africa). The control of Byzantium meant the control of the Black Sea, as well as the nearby Mediterranean Sea. Having such control would lead to commercial and strategic advantages that few other cities around the world had; one could trade via land, but also by sea.

The strategic and commercial value of Byzantium under Constantine, was a redirection of affluence and power from Rome proper.

I think too many people are focusing on the attacks of the Goths. Any care to share why they attacked? How they lived in Roman land and fought in Roman armies before they revolted? I would also say the split did CAUSE the fall of the Western half. Since the Eastern did have a higher population and better trade and took that with them in the split (i.e. more money from taxes and more troops to recruit). Clearly the Romans had little control of the western half of the empire when Rome was sacked in 410 AD. So I am asking how much the economy played a role in the collapse. I think it could be argued as one of the leading causes.

The simple answer to this question is that Constantinople was a much more h*m*genous, ideologically motivated culture than Rome was at the time. Founded by Constantine, the city was intended to be Christian from the start and throughout most of its history it was ruled and populated by dedicated Christians who acted in a relatively directed and coordinated way.

Rome, on the hand, was a fractured society. There were plebs, patricians, foederati, slaves, refugees, Christians, pagans and all kinds of other factions constantly quarreling and fighting without leadership or consensus. The imperial faction of this melting pot eventually ended fleeing to Ravenna, a fortress, and setting up sort of a mini-Constantinople there. Rome completely fell apart after that.

In the "early" going, at least (fourth and fifth century A.D.), part of the differences in the fate of the Roman Empires had to do with the movements of the Goths and Huns. http://en.wikipedia.org/wiki/Goths

"To make a long story short," the Huns chased the Goths out of Eastern Europe and the Balkans (e.g. HUNgary), and these people in turn migrated to, and took over the Italian peninsula and western Europe from the Western Romans. No such thing happened to the Eastern Romans (in modern day "Asia Minor,") and they were spared for almost a thousand years.

The Eastern empire kept more of its troops on the borders, while the Western empire kept more troops close to the Emperor. The East also developed diplomacy to a fine art, which was a bit lost on their Western counterparts, and so was able to survive without being the toughest nation out there by a good margin.

Furthermore, the Eastern empire is often said to have been technologically further advanced, had more population and was richer, but I am not sure if or why this would be true.

That was indeed the case. Eastern Mediterranean ("Levant") in general had civilization superior to the western part of the Roman Empire (btw there was only one Roman Empire - what we call Western Roman Empire is the part of the Empire governed by the Emperor of the West). That started to turn around only around 1000 AD.

In my opinion, the key event for the fall of the West was Vandalic invasion of North Africa in 420s AD which took away critical economic resources from Romans. They were aware of that fact and there were several attempts to recapture Africa. All failed until the expedition of Belisarius in 530s AD.

The slavery hypothesis is weak - all parts of Rome used slaves heavily.

When Rome was a unified entity, it could use all its resources on a threatened frontier to restore the situation, wherever the problem was. This is one of the reasons that the recovery from the third century crisis was so rapid - once the issue with defecting armies and usurpers was solved by the Aurelian or the Tetrarchs, apparently huge issues like the threefold split of the Empire could be cleared up amazing quick.

When the Roman Empire was divided, the two halves were not identical. The East was richer, the East had more defensible frontiers. Often Constantinople's foreign policy was to hide and wait for the enemy without the walls to get tired and leave. For the west to do the same would give up Italy to ravaging...as with Alaric before the sack in 410.

The setting up of a capital city and second court in Constantinople after 395 weakened this unity. It became harder, and slower to convince the rival court to dispatch aid. The West had better troops at first (remember, the army routed at Adrianople was the Eastern Army). This advantage was eroded some when Theodosius defeated Arbogast at the Frigidus to re-unite the Empire in 394.

The West's problems after that soon became apparent. It had two huge and long river boundaries to defend, plus whatever armies turned away from the walls of Constantinople on them. These armies needed to be paid, and the cash poor Empire had to trade land to its mercenaries. This solved the immediate problem, but took more land out of the tax base and made future problems worse. The Invasions of the Vandals in 406 and the Revolt of the Goths put two major threats inside the Empire, and the pogrom against the Goths just made their army desert to the enemy.

This downward spiral was never fully arrested, and even the attempts at recovery in the 410s and 420s just led the remnants of the Vandals to invade and take Africa. This cut the heart out of the Western Empire and despite the several attempts to retake the province, partially bankrolled by the East, the loss could never be made good. When this hope was gone, the citizens of the provinces had to see that there was no hope of ejecting the various barbarian settlers, authorized or not and had to make terms with the powers resident there. Eventually, that happened in Italy itself in 476 and it was all over.

There are several answers to that question; various authors have favoured one or another, but it is probable that the fall of the Western Roman Empire was due to their combination.

From a geostrategic point of view, the stability of the Roman Empire was guaranteed by the legions: strong but not numerous forces, able to intervene in many places thanks to their high mobility. Legions were thus supported by strong logistics (supply chain) and infrastructure (the famous Roman road network). However, the West, being less populated than the East, had comparatively more boundaries to handle with less troops. Moreover, a big part of the Eastern Empire boundaries were shielded against invaders by geography (Black Sea, Caucasus mountains, Arabic desert...) and by the Persian Empire.

In the fourth century AD, due to increasing population, the "Barbarians" were seeking land to settle. From the point of view of a Visigoth, Rome is the Civilization, and room was to be found mostly in the Western part of the Empire. Indeed, all along the third and fourth century, there had been a constant stream of "germanic" newcomers who were willing to become part of Rome, and eager to obtain land, by charity or by force, whichever was necessary. What changed at Adrianople is that the immigrants became too numerous to content with henceforth unoccupied land, and they turned out to be too strong to be dealt with militarily. Adrianople is the symbolic pivot because in the aftermath of the battle, Rome had to evict Roman citizens to meet the demands for land of the invaders. This is the point where Rome failed to defend its own. This is the moment where people began to cease to believe in Rome.

Demographically speaking, the fall of Rome, or at least part of the Empire, was "unavoidable" (with all the usual caveats in these matters), since the population in the Empire was somewhat constant, while it was rapidly increasing in northern and eastern Europe. That the West half should fall first was "logical" because it was larger, less densely populated, less supported by its economy and infrastructure, and primarily targeted by invaders.

(Edward Luttwak's books are a very good reading on the strategic aspects of the Roman Empires, both before and after the split: The Grand Strategy of the Roman Empire and The Grand Strategy of the Byzantine Empire.)

These considerations must not hide other facts:

• From the third century onwards, a constant theme in the Roman Empire was regular usurpations. Most emperors got the job by being proclaimed by their enthusiastic legions; when such a general was given this "nice" present by his troops, he had little choice but to roll with it and try to defeat the other legions still loyal to the previous emperor. If he succeeded, then he knew that he would have to crush many similarly rebellious commanders. The Roman military forces were mostly employed to quell such unrest.

  In the late fourth century AD, it had become quite impossible for distant provinces to obtain any form of military help from Rome. This is the meaning of Rome's evacuation of Great Britain in 410 AD: as Honorius puts it, Britain's cities had to fend for themselves now. This was what they had already done for some time: legions were sent only to suppress rebellions. In that sense, the Western Empire can be said to have fallen by fighting itself to death.

• Germanic people were increasingly part of the legions themselves. Rome was finding expedient to hire these newcomers, who were more easily expendable than Roman citizens. Great Germanic generals like Stilicho and Odoacer were really considering themselves as "Romans". From their point of view, they did not "invade" the Empire; they were the Empire.

• It has been argued that a severe and long-standing economic crisis had made the Empire unsustainable. The roots of that crisis have been variously attributed to demographic stagnation, rarefaction of precious metals, disruption of long-distance commerce... even epidemics, introduction of rats and climatic changes have been invoked. In fact we don't really know; but we can see, for instance, that the population of Rome (the city) had been steadily decreasing, from a maximum of about 1.65 millions in around 100 AD, down to 1.1 millions in 400 AD, then sharply falling to 0.5 millions in 450 AD, then 100000 in 500 AD. Though the big drop is contemporaneous with the formal "fall of Western Empire" of 476 AD, the problem was apparently much older.

  Therefore, it may be so that Odoacer, once having grabbed the reins of power, was lucid enough to realize that the former model had outlived its usefulness, and had to be dropped. Such economy-related explanations may make sense only if we can explain why the East was able to pull through; since climate, agriculture and commercial networks in the East used different structures, this kind of explanation is possible.

• Some authors have blamed Christianism, preventing proper assimilation of Gothic people because they are Arianists, due to an historical accident: Ulfilas converted them to Christianism just at a time when Arianism was still fashionable. Goths and Lombards have clung to their now heretic liturgy because it was in their own language. The intransigence of Catholic rulers would have triggered rebellions, then chaos and fall.

  This is too simplistic a story to be accurate. However, it highlights the fact that the society structure was changing. Indeed, the Germanic people were trying to become Roman, but not the same kind as the Romans already there; they still wanted to retain some of their identity. In very anachronistic terms, we might say that the new immigrants were not content with a central government, and were pushing for diluted federalism.

Lars Brownworth discusses the survival of the Eastern Empire and by tangent the fall of the West in "Twelve Byzantine Rulers" in Episode 5: Zeno. His book by the same name presumably discusses the same. The podcast discusses the general situation at the time of the various emperors essentially being puppets of barbarian generals and the like. The fall of the West wasn't some immediate thing.

The last emperor was deposed but it wasn't until later when no new Emperor was crowned that the empire "fell". It was more that was the end of direct Roman control over the remains of the Western Empire. The Eastern Empire avoided this fate by the work of Zeno who managed to throw off the barbarian yoke in the east and forged a solid state that proved durable enough to survive as a united thing.

Odoacer and later Theodoric both payed homage to the Emperors in the east but after Theodoric died, the later kings and chiefs in Italy didn't mint imperial coins or do otherwise to show any kind of fealty to Constantinople. This eventually prompted Justinian to invade Italy.

The biggest difference between the military threats of the Goths and the Huns compared to Persia was the migratory nature of the former versus the centralised (and thus spatially constrained) government of the latter.

Rome and Persia had sparred against each other in the mesopotamian region for centuries, but, though one or the other might gain ascendancy, they were unable to maintain their advantage beyond their centre of gravity.

In contrast, although germanic tribes had been fought off by the legions repeatedly over the centuries, when the Goths did finally break into the Empire, they brought their entire nation with them. It took just one sustained failure to deal a fatal blow to the Empire.

Note that one of the reasons for the continued ebb-and-flow of the balance of power between the Persians and the Romans was that the Persians were also having to hold off similar migratory opponents on their north-east border (not always successful, as illustrated by the Parthian rule between the two Persian empires).

In this sense, Persia acted as a buffer for the Eastern Roman Empire, leaving only a small part vulnerable, north of the Black Sea. A good reason for the quality of the defenses of Constantinople!