What caused the fall of the (Western) Roman Empire?

I've gotten into several debates recently about what the causes were of the structural weakness inside of the Empire that rendered it unable to withstand the migrations of the 5th Century. It seems to be generally concurred that a stronger Roman state had survived objectively greater crises before (see: Crisis of the III Century), and could have done so again, but for various internal weaknesses in the army, government, and civil service.

So, what were ultimately the causes of those weaknesses?

I've seen it argued that the Christianization of the Empire was what rendered it unable to survive, making the argument that Christians were less willing to fight for the Empire or otherwise. However, I don't buy this for several reasons. Firstly, the East--the most heavily Christian part of the Empire--survived much, much longer. Secondly, the Barbarians who set up successor kingdoms (rather than transitory peoples like the Huns) were almost always Christian, with the exception (the Saxons, Angles, and Jutes) colonizing an extremely peripheral province that Rome made essentially no effort to defend in the V century.

I would instead make the argument that the genuine cause of this institutional collapse was climate change leading to economic decline. It has been found by modern scholarship that there was a major cool period between roughly the Fifth and Eighth centuries CE. This may have played a role in precipitating the Migrations, but also hindered the economic prosperity of the Empire as agriculture became less productive. To counter this, later emperors began introducing the Colonate and other early forms of serfdom, both binding Citizens to the land (and thus preventing them from being soldiers) and reducing their stake in the Empire to the point where they didn't really care if it was a Roman or Barbarian ruling them, or in some cases actually preferred Barbarian rule to escape hereditary obligations. Ultimately, this stress was one the Empire could not handle.

(On a side note, I think that this points to what was exceptional about the Roman state and what gave it its longevity, it's ability to for much of its history give the ordinary citizen a stake in the continued prosperity of the Empire without falling too much into mob factionalism and short-term goals. I'm not saying that it was a model society, nor that democracy would be unsustainable either then or now, but rather that the Roman model was very well suited for an extremely large and diverse empire. Once this broke down, at least in the West, the Empire collapsed).

So, what do you all think was the primary cause, if any, of the Imperial collapse?
 
I have my doubts on Christianization being a factor in the decline of the Roman Empire. I would only agree to this point if there was a rivalry between the Emperor(s) and the Bishop of Rome. You could say it helped the Germanic Tribes and Romans to come closer. If the Empire was lead properly it could have major advantage because of it.

The Western Part, IMO, the declined due to lack of strong resources for a large state which is required. Not enough money or manpower to defend the borders. That is reason one. But there are more and as important as the first one. Pretty useless emperors. Honorius, Valentinus III etc were bad emperors although Majorian tried to save the Empire as much as possible. The rivalry of the emperors with strong figures like Stilicho and Aetius did not help either.

At last, the Huns were also a major factor. You could say, the Mongolian invasion alike, further in the west in the late Classical Age.

When the Emperor is bad, one needs some to compensate his non existent skills. Strong figures or maybe even a strong Senate (the latter is debatable as it may trigger civil war in the state).
 
I know this is probably a cliché, but I think it's that Rome's elites grew complacent and, therefore, didn't put in the necessary effort to save the Empire. Until literally a few years before Romulus Augustulus was deposed, you had people confidently expressing the belief in Rome's eternal glory and greatness. But if the Empire's going to last forever anyway, why bother paying your taxes? Why bother carrying out your duties faithfully, when you could just embezzle money for your own ends? This problem seems to have been most acute in the West for whatever reason -- I forget the exact figures, but Williams and Friell's The Rome that did not Fall has statistics indicating that tax evasion and embezzlement were much more widespread in the Western Empire than in the East. The Western Empire was vastly more wealthy and populous than any of the barbarian tribes, but because its wealthy citizens didn't pull their weight the government was simply unable to make effective use of these resources.
 
Overall, I do agree with your reasons, but I’d like to expand on them a little starting from the immediate reasons and going back from there.

First off, I believe that the WRE, with all its weaknesses and faults, could have survived a bit longer than it did, if it had managed to recover Africa from Gaiseric. It would have taken away the Vandals’ power base, it would have restored the much needed crop yield for the remaining territories and it wouks have aided the empire to slightly recover financially. None of that happened, however, since despite multiple attempts, the Romans never managed to land their troops in Africa, either due to sheer incompetence (curtesy of Basiliscus), misfortune (Majorian) or unwarranted distractions (Attila’s invasions). As far as it goes for the foremost immediate reason for the Empire’s collapse, I’d say Gaiseric and his Vandals had more to do with it than any other barbarian leaders, including Attila.

This is not to undermine the role played by the Huns in general in ushering the Empire’s end. But their role in it rests more on pushing the massive migrations that would eventually pour into the empire’s borders, rather than their direct attacks to the empire. The Huns failed in establishing a firm presence within the empire, and after Attila’s death, they effectively collapsed, with other tribes profiting from the voi of power to lead further devastating attacks on the empire, now deprived of Aetius, its most capable defender.

On that note, Valentinian III also played a big part in the Empire’s collapse. As Constantius III proves, in the first half of the fifth century it was not yet too late for a strong emperor to impose himself and successfully attempt to restore the empire’s fortunes. Had you asked any Roman prior to 422 CE, he would have told you that things were going just fine for the empire, and they were, to a point. However everything soon went in disarray when Constantius died, followed shortly after by the weak willed and lackluster Honorius. After a much disputed succession, Valentinian III rose to the throne. Granted, he was young when he did, so there was not much he could. Growing up, however, he turned out to be the worst kind of weak, the one who’s both stupid and vicious. He did nothing worth remembering for most of his life, and when he finally did, it was the worst possible thing he could do. He killed Aetius, the most capable general the empire had, only to be dispatched shortly after in a palace coup. In the ensuing chaos, the Vandals sacked Rome, and Unlike the Visigoths 45 years earlier, the Vandals were not polite about it at all. After that, the empire had almost definitely reached its twilight, Majorian rose to the fore and attempted to be the man Valentinian III would have needed to be to save the empire, but by then emperors were owed no real loyalty, so once he lost the fleet he was meant to attack the Vandals with, he became easily disposable.

Weak emperors aside, the inherent weaknesses of the WRE could have been avoided, however, if it had been as economical strong as the ERE. The two halves of the empire often collaborated, and apart from some frictions, there was no real enmity between the two courts. However, unlike previous divisions of the empire, this one had real and substantial formality behind it, resources weren’t shared unless by previous agreement , administration was effectively independent, and if one half decided to not collaborate with the other, the second one was left to its own devices until diplomatic overtures were made to fix things. This deprived the WRE of the substantial economic wealth of the ERE, of its manpower and of its grain, so that emperors in the West had to rely more and more upon wealthy landowners to supply them with money and men for the army, for economical reasons analyzed below. Unfortunately, said landowners had no intention whatsoever of paying taxes and giving up men for conscription, so they paid the emperor, or whoever was in charge, with enough money to buy the services of Barbarian mercenary forces, which, despite everything, was not necessarily bad, if they also didn’t complain about the army being too Barbarian for their tastes, threatening to withdraw their support of whoever was in charge unless the army was made more Roman. It wasn’t only Senators, however. Commoners were increasingly becoming less loyal to the empire’s defense, at times even aiding the barbarian’s assault, since they felt less keen on defending a political entity that more often than not neglected them for whatever reason. Thus, the division of the empire, and the Romans themselves, are to account for the WRE’s failure to survive.

To go further in the economical aspects, climate change did play its part, but that’s not all of it. The WRE’s economy mostly depended upon the massive estates of wealthy senators, which were based upon their villas. These villas, however, depended upon soldiers in their immediate vicinity to function. When the WRE began arrenging for the barbarians to settle in the borders to defend them, the villas lost some of their source of revenue, since Barbarians provided for their own supplies, and, in any case, they could not always be trusted to not despoil the land, so most senators either moved closer to Italy, or in what became far flung provinces, and Britain and Spain. Britain landowners then, since the empire had effectively abandoned them, stopped paying their taxes, and the ones in Spain, poorly defended, saw themselves embroiled in a civil war which involved a usurper, the usurper’s wannabe usurper, and hordes of Vandals, Alans and Suebi. Order was restored there, only to be lost in Africa, which, for the reasons stated above, was to be the determining factor in the Empire’s collapse. To all this, it surely didn’t help that Roman currency stopped being strong and stable since the middle of the third century, when emperors, forced by their soldiers, felt forced to debase silver coins more and more to meet their demands. Notably, it was the one problem of the empire that Diocletian attempted and failed to solve with his edict on prices.

Landowners, as already mentioned, were also necessary to supply the army with men. When pinpointing the most devastating battle for the Romans in the fourth century, the first answer that comes to mind is Adrianople in 378 CE. The battle was definitely a huge Roman loss, an entirely avoidable blunder on Rome’s side, but it wasn’t such a huge disaster that the empire couldn’t recover from it. Contrary to popular belief, the Roman army was still the most effective fighting force of antiquity, and the Goths actually failed to follow up to their victory, signing a deal with Theodosius to settle in the borders at the Balkans and fight for him whenever asked. What was truly devastating was the loss of manpower the western half of the empire incurred in the battles of Siscia (388), Potovo (388) and Frigidus (394). Losses were so heavy that Stilicho, a decently capable general, almost never felt confident to risk pitched battles in his period of regency of the West, and was increasingly forced to let barbarians defend the Empire’s borders, which led to the collapse of the villa based economy, the disillusionment and disloyalty of the empire’s inhabitants, the detachment of provinces who felt ignored by central authority, and further pressure from Barbarian tribes, either interested in gaining a position within the Roman army, or simply in settling within the empire.

Lastly, I agree with you that Christianity had little to do with the empire’s collapse. Christian senators were as loyal as pagan senators, for better or for worse.

To sum it up, my reasons for the fall of the WRE are: Gaiseric and the loss of Africa, Hunnic invasions and increased pressure from barbarians, economic crisis, lack of manpower, weak central authority, the empire’s division, and the forced reliance on Barbarian troops due to the Romans themselves not effectively contributing to the Empire’s defense.
 
@Sertorius126 this reminds me of the fall of the Byzantine Empire in lots of ways. Aristocrats not paying any tax. Debasement of the currency. Citizens preferring foreign rule. Incompetent rulers. Migrating tribes at the border. Loss of control over the economy. Increased reliance on unreliable mercenaries. Lack of indigenous military strength.

All this applies to the period 1282 - 1355, which saw the total collapse of Byzantium from regional power to tiny city state. Parts of the above list had been true to some extent earlier, at times in the 11th and 12th centuries, but it was in the 13th and 14th centuries that they really became a problem.
 
@Sertorius126 this reminds me of the fall of the Byzantine Empire in lots of ways. Aristocrats not paying any tax. Debasement of the currency. Citizens preferring foreign rule. Incompetent rulers. Migrating tribes at the border. Loss of control over the economy. Increased reliance on unreliable mercenaries. Lack of indigenous military strength.

And the ever present penchant for civil strife. History really does repeat itself.
 
The Western Roman Empire had the same or larger population than the East. They had better agricultural land, though the climate change speculated about may have nullified that. Most historians think the Western army was tougher than the eastern one, there was less religious controversy than in the East, and they didn't have to deal with the Persian empire. They didn't have Constantinople, but could have done more in terms of fortifying the Alps and Revenna, the capital, was really hard to take.

So you could have had a scenario where the West survived and the East fell. In China, about the same time, the more legitimate Chinese dynasties held on in the less developed South, while it was the wealthier North with the older civilization that had the nomad successor states, the opposite of the situation in Europe.

Here were the four big problems with the West compared to the East:

1. Weak Emperors, much worse leadership. The Roman (and later Byzantine) system could not handle several weak and/ or short-lived Emperors in a row, as had been demonstrated in the crisis of the third century. Over the course of the fifth century, the leadership of the Eastern Empire tended to improve while that of the West deteriorated.

2. Climate change and plagues probably played a much better role than has been recognized.

3. Aristocrats not paying tax, with the aforementioned weak Emperors let them get away with.

4. The rulers of the East massacred their German mercenaries and found other sources of manpower for their army. The ones in the West wouldn't or couldn't do this.

In fact any of these four reasons would be sufficient.
 
Many good points have been brought up, so there's not a whole lot else I can add that hasn't already been said but here goes.

The real tipping point, in my opinion, was the granting of land and legitimate commands to Germanic leaders within the army and government. The Roman citizenry showed remarkable loyalty towards the imperial government for more than five centuries, and a big part of that was the overwhelming combat capability of the Roman army. However, once barbarian forces posed a great enough threat that the imperial government attempted to co-opt their military capabilities, all of the citizens' patriotism fell away because the imperial government was functionally hanging them out to dry because it was more financially reasonable to rely on barbarian troops in some quarters. The role that the loss of Africa to the Vandals played in the financial underpinning of the WRE's security issues is debatable (Africa was certainly not the only productive farmland in the Western Empire), and some blame can perhaps be placed on Stilicho for his impromptu distribution of limited manpower to non-crucial fronts. He certainly delegated significant troops to Illyria as part of a game of brinkmanship with the ERE over control of Greece that may have been more useful defending the Rhine or containing the Goths.

Ultimately, there's no singular answer to the question of why the WRE fell, and I think that's probably due to the very limited number of primary sources from this time period. I think as the fields of environmental archaeology and history gain acceptance in mainstream historical literature, there will be more light shed on the ecological underpinnings of the collapse (and whether or not the climate changing was an impetus for the migrations or any related financial woes).
 
The Western Roman Empire had the same or larger population than the East. They had better agricultural land, though the climate change speculated about may have nullified that. Most historians think the Western army was tougher than the eastern one, there was less religious controversy than in the East, and they didn't have to deal with the Persian empire. They didn't have Constantinople, but could have done more in terms of fortifying the Alps and Revenna, the capital, was really hard to take.

So you could have had a scenario where the West survived and the East fell. In China, about the same time, the more legitimate Chinese dynasties held on in the less developed South, while it was the wealthier North with the older civilization that had the nomad successor states, the opposite of the situation in Europe.

Here were the four big problems with the West compared to the East:

1. Weak Emperors, much worse leadership. The Roman (and later Byzantine) system could not handle several weak and/ or short-lived Emperors in a row, as had been demonstrated in the crisis of the third century. Over the course of the fifth century, the leadership of the Eastern Empire tended to improve while that of the West deteriorated.

I am not sure weak is the best term here, the growing lack of a central authority led to a continuous civil war. Towards the end, the fighting was purely over who would be on top. The empire's resources were used as one general fought another to get to the top.
 
Paraphrasing my history professor:

As the empire grew it became more and more east-oriented, it was where most major cities were, where most trade happened and importantly; where most enemies were. This meant that the east became the empire's #1 priority with the west being far behind, almost all legions were deployed in the east to fight the Persians.

Unfortunately the presence of military forces was kinda required for the collection of taxes and taxes for the functioning of the state. The more soldiers were shifted to the east, the less taxes could be collected in the east, the less valuable the land became to the empire; and thus further focus was put on the east.
 
Many good points have been brought up, so there's not a whole lot else I can add that hasn't already been said but here goes.

The real tipping point, in my opinion, was the granting of land and legitimate commands to Germanic leaders within the army and government. The Roman citizenry showed remarkable loyalty towards the imperial government for more than five centuries, and a big part of that was the overwhelming combat capability of the Roman army. However, once barbarian forces posed a great enough threat that the imperial government attempted to co-opt their military capabilities, all of the citizens' patriotism fell away because the imperial government was functionally hanging them out to dry because it was more financially reasonable to rely on barbarian troops in some quarters. The role that the loss of Africa to the Vandals played in the financial underpinning of the WRE's security issues is debatable (Africa was certainly not the only productive farmland in the Western Empire), and some blame can perhaps be placed on Stilicho for his impromptu distribution of limited manpower to non-crucial fronts. He certainly delegated significant troops to Illyria as part of a game of brinkmanship with the ERE over control of Greece that may have been more useful defending the Rhine or containing the Goths.

Ultimately, there's no singular answer to the question of why the WRE fell, and I think that's probably due to the very limited number of primary sources from this time period. I think as the fields of environmental archaeology and history gain acceptance in mainstream historical literature, there will be more light shed on the ecological underpinnings of the collapse (and whether or not the climate changing was an impetus for the migrations or any related financial woes).

In defense of Stilicho, his main objective was to put pressure on the Eastern court so that he would be recognized as “parens” for both emperors. Having both halves in his control would have consistently helped him in better defending the empire. His scheme failed repeatedly, however, and over the years he was lacking more and more in manpower and money, so eventually he attempted to seize the whole prefecture of Illyria in a quick campaign out of desperation. Ignoring Constantine III for that was a blunder with catastrophic results, but I can sympathize.
 
What caused the fall of the Western Roman Empire?

One interpretation of the fall of the Western Roman Empire is that it was ultimately caused by economics.

The west seems to have been suffering from de-urbanisation even before the collapse. It has been stated that this may have been partly due to the reforms of Diocletian IIRC, which stripped away the incentive for nobles to invest in urban centres and sucked the life out of towns and cities. There also may have been a significant population decline, judging from the reduction in evidence of trade activity, coinage and other indicators used in archaeology.

The debasement of the coinage is another key problem. The high-quality of the empire's coinage has often been seen as an indicator of financial and economic health. But in the third century and onwards, the currency was repeatedly debased, at times reaching the point where it became virtually worthless. The price edicts were another costly failure.

If it costs me $5 to produce a unit of milk and bring it to market, but the government has set a price edict which says that I can only sell the milk for $1, then what do you think is going to happen? Am I likely to sell the milk at a loss?

Of course not. What actually happens is that I'll either go out of business, or I'll go underground and join the black market. In the case of the former, goods simply disappear from markets, causing dislocation, chaos and economic collapse. In the case of the latter, some level of activity continues, but it becomes more expensive, less reliable, and quality goes down, and on top of that no tax is collected by the government.

On top of that one can layer climate change. This seems to have prompted the Huns to migrate, which set in motion the Germanic tribes. This created enormous pressure at the Roman frontier, which Rome couldn't stop. Combine this with the fact that several hundred years of contact with Rome had caused Roman agricultural techniques to spread into Germania, and the fact is the German population had expanded enormously over time. Add to that, the political pressure of dealing with constant Roman intervention led by a process of evolution to much larger Germanic kingdoms forming at the frontier, who possessed the strength of numbers to successfully defend their interests and increasingly challenge Rome.

This last point was made by historian Peter Heather in his excellent book about Rome, The Fall of the Roman Empire - A New History.

The practice of increasingly using Germanic troops didn't help at times, although the real problem was more that Rome treated them badly (such as massacring the soldiers' families in pogroms), which led directly to the Sack of Rome in 410.

The Bagaudae (rebels) seem to have been an increasing issue at this time as well. Bands of free people/road robbers/outlaws roamed the countryside, causing disruption and contributing to the collapse of the west. Little is known about them, but they seem to have been a symptom of the increasing societal collapse and failure of the Roman system to serve the needs of the people it was supposed to protect.

It's a similar story to how the later Byzantine Empire lost so much land so quickly, first to the Arabs (religious persecution of eastern Christians and Jews led many people to conclude that Arab rule was preferable), and then later to the Turks (when John II Komnenos campaigned in Anatolia, there were local Greeks who fought ferociously on the side of the Seljuk Sultanate against the Byzantine army, on the grounds that they preferred the Turkish rule to Constantinople).

When a large enough portion of an empire's citizens decide that they are better off under a different system and would rather join the empire's enemies than fight in its defence, the game is up. It's over. It's time for a new system to emerge.
 
I believe North Africa is being extremely overestimated... It may have provided crop yields in 0 AD when the climate was warmer and more conducive to agriculture. However, the life is being much more tedious and by the Islamic period, was already beginning widespread desertification. If Rome plans to fight to keep it, it will cost exorbitant amounts to irrigate.

It is more beneficial to maintain Gaul than it is Africa, at least in the long term agricultural benefit.
 
I believe North Africa is being extremely overestimated... It may have provided crop yields in 0 AD when the climate was warmer and more conducive to agriculture. However, the life is being much more tedious and by the Islamic period, was already beginning widespread desertification. If Rome plans to fight to keep it, it will cost exorbitant amounts to irrigate.

It is more beneficial to maintain Gaul than it is Africa, at least in the long term agricultural benefit.

Interesting post, this got me thinking about geography and climate, an aspect of history that I often find interesting.

I did a bit of hunting around, and found this map. In the first image, it shows the countries that produce enough food currently to meet their needs.

erl452631f1_online.jpg


In the second image, it shows the countries that could produce enough to meet their needs, if they chose to allocate the necessary resources to it.

What's interesting is that either way, Morocco, Algeria and Tunisia would not be able to provide sufficient food to meet their needs even if all available land and water resources were used to produce food.

One caveat here is that the map assumes these efforts are made with current productivity levels in each country. So in theory, if productivity could be improved (say by wide-scale irrigation, use of fertilisers, water conservation methods, and increased mechanisation), it might be possible to produce more. Morocco was one of the countries that increased its cereal yield by the highest percentage (Nearly 10%) between 2010 and 2018.

Even so, it's generally not looking great for North African countries compared to the rest of the world.

Now, of course that's just the modern day. Conditions could have been/probably were more favourable in the past. Deforestation hasn't helped, nor has desertification, nor globally rising temperatures.

Just thought somebody might find it all as interesting as I did! :)
 
Interesting post, this got me thinking about geography and climate, an aspect of history that I often find interesting.

I did a bit of hunting around, and found this map. In the first image, it shows the countries that produce enough food currently to meet their needs.

erl452631f1_online.jpg


In the second image, it shows the countries that could produce enough to meet their needs, if they chose to allocate the necessary resources to it.

What's interesting is that either way, Morocco, Algeria and Tunisia would not be able to provide sufficient food to meet their needs even if all available land and water resources were used to produce food.

One caveat here is that the map assumes these efforts are made with current productivity levels in each country. So in theory, if productivity could be improved (say by wide-scale irrigation, use of fertilisers, water conservation methods, and increased mechanisation), it might be possible to produce more. Morocco was one of the countries that increased its cereal yield by the highest percentage (Nearly 10%) between 2010 and 2018.

Even so, it's generally not looking great for North African countries compared to the rest of the world.

Now, of course that's just the modern day. Conditions could have been/probably were more favourable in the past. Deforestation hasn't helped, nor has desertification, nor globally rising temperatures.

Just thought somebody might find it all as interesting as I did! :)

I’m skeptical regarding the heat as a reason for the change in productivity. The height of North African farming existed at the warming periods and then declined following the Islamic conquests which coincided with this cooling period that continued until the high Middle Ages and would resume later in the 14th century and the later ‘little ice’. However, I’m not familiar with this topic, but it is clear that Rome will not save itself in the 6th or 7th century relying upon Africa and moving forward, Africa becomes even worse of an efficient breadbasket. Egypt is the only worthwhile acquisition in the long run in this region and even then it experienced declines.
 
This is an exceptional complex and controversial subject about which historians have debated for centuries. I shall try to give you my thoughts on this matter.

First there are a couple of ideas that are thrown around above, that have largely been rejected by modern scholarship.
Christianity: Gibbon is the most famous proponent of this argument (so its an old argument). There are a couple of key problems with this argument. First the timing does not really work. The Empire largely recovered from the crisis of the 3rd century at the same time it rapidly Christianized. Further, the Eastern Empire continued for another thousand years.
Debasement of Coinage: This is a superficially plausible theory but here the timing really does not work. The debasement occurred in the middle of the third century. The coinage was restructured in the time of Diocletian and the Empire survived for another 2 centuries or so. Further there is little evidence that trade declined during this time. In the same way Diocleatian's edict on maximum prices does not work in terms of explanation. Moreover, most evidence suggests it was not really followed.

Did the Western Empire Fall: This remains a popular idea amongst some academic historians. The basic idea is that the Western Empire did not really fall; rather it transformed into something different (medieval civilization or something). I will admit that I do not have deep expertise in this strain of thought but its strikes me as profoundly implausible. By any reasonable measure the distance people travelled and the distance goods travelled and basic measures of the standard of living (e.g. soot and lead particles in ice core of Greenland) dropped dramatically after around 450 CE. The sense I get is that this idea is declining in popularity in academic circles.

So what did happen?

The Climate Change/Barbarian Invasion Hypothesis (most often associated with Peter Heather) does seem to have some plausible elements. In particular the timing works well. The Huns pushed the Visigoths into the empire. The Visigoths destroyed the army of Valens in 378 at Adrianople. The Visigoths sacked Rome in 410. The Rhine froze in 406 and allowed the Vandals to cross and eventually take North Africa. Ultimately, however, I find the explanation insufficient. For example, it ignores the question of why the Empire (or earlier the Republic) was able to deal with large barbarian invasions (e.g the Marcomani in the time of Marcus Aurelius, the Germans in the time of Gaius Marius). Therefore, I tend to put the Huns/Barbarians as contributing but not causal factors.

Instead I tend to support the idea that several factors converged to bring about the fall. First, there were a series of child/incompetent Emperors for nearly a century (Honorias, Valentinian III etc.) This weakened the ability to deal with the Barbarian invasions. Further there emerged at this time an intense suspicion of people with barbarian ancestry. For this reason many competent men (Stilicho for example) could not rise to the purple. This actually marks a sharp departure from traditional Roman practice. Why was Stilicho less Roman than the Illyrian emperors (i.e. Aurelian, Dicoletian, Constantine). Strong German Emperors may have made a difference. The weak Emperors in turn led to the collapse of trade which further weakened the Roman state and in particular (and in combination with the disaster at Adrianople) rendered the maintenance of a standing army impossible. I think the decline of trade is one of the most disastrous aspects of the fall in that it become autocatalytic. For example, losing North Africa reduced trade between North Africa and Italy which dramatically weakened the state. The weakened state could not pay the army which caused the loss of more Gaulic provinces which further reduced trade. Finally, there is just some "bad luck". For example, if the fleet had not burned and the Empire had retaken North Africa this may have stimulated trade and allowed the maintenance of the army reversing the decline.

Ultimately a complex question which historians will continue to debate for centuries.
 
The west seems to have been suffering from de-urbanisation even before the collapse. It has been stated that this may have been partly due to the reforms of Diocletian IIRC, which stripped away the incentive for nobles to invest in urban centres and sucked the life out of towns and cities. There also may have been a significant population decline, judging from the reduction in evidence of trade activity, coinage and other indicators used in archaeology.

The debasement of the coinage is another key problem. The high-quality of the empire's coinage has often been seen as an indicator of financial and economic health. But in the third century and onwards, the currency was repeatedly debased, at times reaching the point where it became virtually worthless. The price edicts were another costly failure.

The de-urbanization and population decline were likely the fault of climate change, since it lead to decreasing agricultural productivity and in turn a lower per farmer surplus requiring a greater share of the population to work the land.

As for currency debasement, I don't really buy this argument, because 1) the Edictum de Pretiis often wasn't enforced and 2) was repealed over 150 years before the fall of the Roman Empire.

I’m skeptical regarding the heat as a reason for the change in productivity. The height of North African farming existed at the warming periods and then declined following the Islamic conquests which coincided with this cooling period that continued until the high Middle Ages and would resume later in the 14th century and the later ‘little ice’. However, I’m not familiar with this topic, but it is clear that Rome will not save itself in the 6th or 7th century relying upon Africa and moving forward, Africa becomes even worse of an efficient breadbasket. Egypt is the only worthwhile acquisition in the long run in this region and even then it experienced declines.

The climate change began in earnest in the 4th and 5th centuries, and as you point out it was cooling. Historically, cool periods are associated with dry periods, because more water ends up frozen in ice caps (though that isn't really comparable to modern global warming, which is happening at an astronomically fast rate). That being said, as is already mentioned, holding onto Africa is necessary not only for the decreasing agricultural surplus it provides, but also for trade and to secure Italy's southern flank.

The Climate Change/Barbarian Invasion Hypothesis (most often associated with Peter Heather) does seem to have some plausible elements. In particular the timing works well. The Huns pushed the Visigoths into the empire. The Visigoths destroyed the army of Valens in 378 at Adrianople. The Visigoths sacked Rome in 410. The Rhine froze in 406 and allowed the Vandals to cross and eventually take North Africa. Ultimately, however, I find the explanation insufficient. For example, it ignores the question of why the Empire (or earlier the Republic) was able to deal with large barbarian invasions (e.g the Marcomani in the time of Marcus Aurelius, the Germans in the time of Gaius Marius). Therefore, I tend to put the Huns/Barbarians as contributing but not causal factors.

I think part of it has to deal with climate change, and part of it is sociological. The warm period that coincided with Rome's rise and golden age was extremely conducive to high agricultural surpluses, which enabled the Roman State to grow, but the subsequent cooling of the decline lead to deurbanization as already mentioned that hurt the Roman economy and forced the state to take increasingly draconian measures to ensure it received a high enough portion of value output to equip and supply an army.

This, of course, feeds into sociological factors. As I mentioned in the OP, the Roman State in my opinion was successful because of the process of Romanization, wherein it was able to bring actual benefits to commoners and even conquered peoples in the form of security and increased trade. Whenever these two processes broke down, people lost those benefits and stopped caring about Rome, when coupled with climate change leading to decreased surpluses leading to deurbanization and serfdom, they likely would actually prefer to be freemen under German rule than coloni under Rome. Thus, in the 3rd Century to some extent and then much more in the 5th century, the Western Emperors completely lost the confidence of the people. The fact that they were incompetent didn't help matters, but ultimately it would have required rolling all sixes several times in a row to survive the upheavals of the period.
 
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