crisis of the 3rd century, the empire was already terminal. It continued on, but it was no longer the entity that it once was. When Emperor Diocletian established the Tetrarchy, it was an effort to try and make the empire more manageable, essentially splitting them up into separate entities (Imagine the US being split up into seperate administrative districts of Pacifica, New England, Dominion, Midwest, and Texas). The Roman empire had ceased to exist as a single entity, it was now an organization of seperate but cooperative empires, nominally it was one empire, but in practice many. The Roman empire was dead, only the still breathing corpse remained; it simply took a final push from outsiders to do her in.
I say this because Rome throughout history had been beset, bested by, and occasionally sacked by barbaric tribes, such as the celts and gauls, and earlier in their history by the Etruscans and Samnites. Each time the eternal city came back from defeat and soldiered on, eventually reigning victorious and subjugating their attackers. However, what had changed in the millenia was that Rome was no longer a vibrant society; an age old scourge that had beset societies throughout the ages consumed it from within; much like a cancer attacking a persons immune system or termites eating away at a wooden pillar. This cancer was bureaucracy. The empire had always been large, had always had to deal with an occasional uprising, had always been beset by the very problems that appeared to eventually fell it, but the empires ability to deal with these problems effectively and quickly had changed, not the problems themselves.
Where once defense of a region was left to the proconsul/propraetor (governor) of the province and the legionaries (Roman citizen soldiers) and auxiliaries (Non-citizen residents of the empire looking to obtain citizenship) under his command, occasionally with some help from Rome herself in the event of a major crisis. Over the centuries a large bureaucratic organization had sprung up hamstringing the governors ability to govern, and stealing away resources from more productive endeavour.
This bureaucracy had been implemented by the first emperors themselves, to help them assist governance of the empire, and had gradually been expanded as emperors attempted to avoid any extremely popular governor or general from laying claim to the imperial seat. Over the centuries successive coups and civil wars lead the emperor to believe that he needed to exert more direct authority over the empire rather than less (though some Emperors such as Hadrian bucked that trend). Now this is all conjecture, as all hypothesises of what caused Rome to fall are, but I use this example to state my case.
In 212 AD the emperor Caracalla issued an edict that fundamentally changed the empire. The edict was the Constitutio Antoniniana, which declared every freeborn person residing in the empire a Roman citizen with all the rights and responsibilities that it contained. This was a momentous occasion that one, symbolizes how bureaucratic the empire had become, and two, fundamentally changed its social system, and three, laid the seeds for the crises that would beset the empire in the next two centuries.
The reason Caracalla passed the new law was simple, he wished to increase the taxable base of his empire. The Roman empire was very complex and its taxation system equally so, but to summarize it: outside of a census tax and polling tax, each administered by a puppet king or provincial governor, non-Roman citizens were exempt from direct taxation by the empire, i.e Roman tax collectors representing the city of Rome and not the provincial governors who then passed the tax on after collection some of the revenue for their administrations. Roman citizens were subject to direct taxes from the Empire, but were spared the census tax and polling tax.
By passing the law, and making every person in the empire a citizen, he further strengthened an already large bureaucracy. But more importantly, it was why he did so, it was because the tax was to feed the behemoth itself. The bureaucracy had grown so large that the empire was having difficulty funding it with the revenue sources it had at its disposal. The emperor issued this edict, hoping that this move would solve the frequent funding problems that beset the empire during this time; I should also note that currency debasement was going on cocurrently as well. However, this short term solution had dire long term ramifications.
Firstly, it grew the very bureaucracy that was slowly killing the empire. This bureaucracy had gradually, over the centuries, shifted power of action away from the governors of the provinces and more towards the emperor, making the empire as a whole much more vulnerable to incompetence or corruption of the emperor and his aids. Corruption gradually became a widespread problem that left the organization that ran the empire unable to do its very job.
Secondly the edict continued the empires economic stagnation. Adding addition tax burdens to an ever strained productive middle class straing them even further, and as the middle class goes so goes society. When the middle class thrives and is productive society grows. When the middle class is squeezed and disappears a society stagnates. This edict created an additional massive tax burden, which the emperor made even more burdensome by raising the tax rates on the populace, and coupled with an ever debasing currency, made them unable to pay their dues to the state. This began the process of fuedalization of society because as the ability of the common man to pay his debts decreased the likelihood of seeking a wealthy benefactor rose. By the end of the empire the poor plebians had pledge themselves to the wealthy land owners, who had the power and ability to persuade or outright ignore imperial tax edicts, becoming serfs on land that they once had owned and worked for themselves.
Thirdly the new universal citizenship that every person in the empire enjoyed had a major unforeseen consequence on Romes military ability. The legions of Rome had always been the elite force of the military, not the backbone that we imagine them to be simpy because of numbers. Open to citizens of the empire only, it was a lucrative prospect despite the lengthy mandatory service of 25 years. The average lifespan of a legionnaire was greater than that of an average citizen. The pay was very good, and to top it off, retiring legionaries received generous grants of lands in the outer territories. The empire created colonies where retired legionaries would settle, which helped maintain peace and stability in the region, and more importantly for the legionnaire, created an area where they were members of the wealthy landed class. But as I stated, the legion wasn't the backbone of the miliary. Though the legions were numerous there were far too few of them to patrol all of the empire alone. That was the purpose of the auxiliary, which also served a societal reason for existing along with a military purpose.
During the principate and up to the middle of the 3rd century the auxiliary made up 3/5 of the Roman military forces. The pack mules and specialized soldiers of the military; a good analogy would be to think of the legions as construction engineers and the auxiliaries as the field crews. But most importantly, the auxiliary was comprised of non-citizens of the empire, and the auxiliary units were often stationed away from homeland of the soldiers who had enlisted. This helped prevent rebellions, by removing the armed subjects from their native lands, and increased integration amongst the auxiliaries into Roman societs as the reward for 25 years of auxiliary service was citizenship in the empire.
It may be difficult to understand how lucrative a prospect this was, but citizens enjoyed special economic rights and privilages over other subjects. A person could become rich and successful merely as a subjuct, but it was far easier to become rich when one was a citizen. Citizenship could be awarded by the emperor to upstanding subjects or foreigners who greatly helped the empire, but the most common way to earn it was through military service. As I have mentioned before, the Romans were brutally effective in subjugating a population with an iron fist. On the flip side, they created many opportunities for the excellent or ambitious peregrini (foreigners under Roman control) to become standing members of society. Citizenship in the empire opened doors, it allowed for free travel throughout the empire, and the possibility to hold positions of power. The ability to travel freely was especially a big deal. This is still true today, as I know individuals that have acquired American citizenship simply because it offers greater ability to travel into other countries. However, the Constitutio Antoniniana removed a major incentive for members of the empire to enlist into the auxiliary and from it a major source of military manpower.
This immediately created a man shortage problem. The legions alone were not enough to patrol the boundaries of the frontier lands, maintain peace in the inner empire, and build and maintain the vast roadworks that facilitated trade. This self inflicted crisis forced later emperors to seek out barbarian auxiliaries, though a better word would be mercenaries. Essentially the empire had put itself in a position where it was unable to raise its own army, it had to pay for protection.
And this is where it all comes together. The edict removed any real reason for non-citizens to sacrifice for the empire, since everyone was a citizen now, why would they join the auxiliaries? Men still joined the legions, but the legions were expensive to maintain relative to the auxiliary units, further straining the empire that was already spending every coin it had on its massive bureaucracy.
And as the bureaucracy grew, so did the corruption that comes with large organizations that answer to few individuals. It siphoned resources from the middle class, and actively forced out those who showed great ability for fear of losing their little fiefdoms. In the end those with ability, or simply the resources, stuck to their own fortunes and ignored the empire, carving out little fiefdoms of their own. When the west finally fell in 476 AD, the empire had long been dead, the usurping Germanic leader simply decided he did not want to play along with the farce any more.
This is something we ourselves need to think about. Massive influx of migrants, economic troubles, even incompetent leaders are all survivable. Rome had its fair share over the millenia but survived them all. It was only when the bureaucracy became so large that it cannibalized everything productive and valuable that the empire finally succumbed. That is the threat that America faces today. We have dozens of agencies and and departments that need not exist each one vying for money and influence, and each one looking out for its own interest.
It will not be the migrants, nor economic troubles, nor even a foreign power that will lay us low. It will be the very departments and programs that we create to help us rule our nation because the day is coming when we cannot even pretend to pay for our government as it exists now. And when that day comes, if we haven't fundamentally altered how the nation thinks, then we will see our own Constitutio Antoninana moment.