Short Version: The western Roman empire dramatically overextended but would have recovered, even from the invasions except for the folly of slaves(immigrants) and Christianity(Judaism / Marxism / Neo-Marxism / Feminism / Postmodernism), and finally the Islamic invasion and destruction of the civilized worlds.
1 – Overextension of Colonization: Celtic holocaust created the opportunity for north germanic migration, just as defeating Germany and Europe in WW2 left a vacuum for marxism Judaism and Islam today. And leaving African colonialization incomplete left opportunity for Islam.
2 – Overextension of Range – from the Mediterranean coastal army, with naval transport and strike capability – which is cheap – to a primarily territorial military among hostile less developed people that was expensive. In other words, the cost of domesticating (settling, urbanizing, spreading literacy, commerce, and law) Europe was too high for the returns. (just like today)
3 – Overextension of Agrarian Economy – the empire needed industrialization. The greeks failed to reform their economy. The Romans improved administration and organization – second to none
4 – Overextension of Technology – the empire given the technology at the time, which led to corruption, assassinations, and use of force to obtain rents, rather than profitable service of the state, to obtain status. (just like today)
Bad Strategic Response to Overextension – Splitting the empire with the trade wealth in the greek east but the aristocracy in the roman west. Just like we have all these foreign costs in Europe and in the middle east just to protect Europe from oil price catastrophes – that don’t affect us.
Overextension of Elites Necessary To Govern – empire so that the ratio of elites and middle class to conquered and slaves was too high to rule, just like we have too many non-whites to rule today, and the middle east to govern thru Israel and the monarchies today.
Overextension of soldiery beyond supplying demographic – so that the military was no longer ‘citizens’ – but mercenaries – (Just like Hispanics, Blacks, and Muslims in the military today who are there for a jobs program not patriotism and ‘the people’)
Overextension of Immigration (Slaves): slaves were the old world’s version of third world immigrants today. These people lack the values traditions and culture and if brought in sufficient numbers will always rebel against the host population until they defeat them.
Overextension of TOLERANCE: Christianity undermined values like Christianity v2 (postmodernism-feminism-multiculturalism-equalitarianism) undermined our values. They shifted focus from European zest for life and achievement, for ignorance of life and fantasy after death. Christianity without warfare to dominate it, makes men weak. So Christianity prevented the restoration of the roman empire.
Overextension and Vulnerability to Plague: The soldiers brought back either measles or smallpox or the plague from the middle east and decimated the population.
Fragility to Shocks: The Muslim conquests destroyed the economies of the developed world, profit from conquest was exhausted by 1100, and islam’s mandatory ignorance reduced the developed civilizations to ashes.
Judaism to undermine. Christianity to weaken, Islam to destroy the cancer of the abaramic religions – the war on civilization, evolution, and life itself.
The answer for Rome and for America (europe) is the same: ethno-nationalization of the organs of state and organs of the state, and finance, at all levels, and specialization in running the state – not just here but anywhere we can function as a military, administrative, judicial elite (without outbreeding).
Gibbon gave a classic formulation of reasons why the Fall happened. He gave great weight to internal decline as well as to attacks from outside the Empire.
—“The story of its ruin is simple and obvious; and, instead of inquiring why the Roman empire was destroyed, we should rather be surprised that it had subsisted so long. The victorious legions, who, in distant wars, acquired the vices of strangers and mercenaries, first oppressed the freedom of the republic, and afterwards violated the majesty of the purple. The emperors, anxious for their personal safety and the public peace, were reduced to the base expedient of corrupting the discipline which rendered them alike formidable to their sovereign and to the enemy; the vigour of the military government was relaxed, and finally dissolved, by the partial institutions of Constantine; and the Roman world was overwhelmed by a deluge of Barbarians. … (AND) … As the happiness of a future life is the great object of religion, we may hear without surprise or scandal that the introduction, or at least the abuse of Christianity, had some influence on the decline and fall of the Roman empire… the soldiers’ pay was lavished on the useless multitudes of both sexes who could only plead the merits of abstinence and chastity… If the decline of the Roman empire was hastened by the conversion of Constantine, his victorious religion broke the violence of the fall, and mollified the ferocious temper of the conquerors.”—