There was a lot of good discussion centering around slavery and its negative effect on technical innovation. After all, why would you need automation to increase the effectiveness of farming or other industrial work if you can just buy another thousand or even ten thousand war slaves for use as laborers on your farm or in your copper mine? Sure, they cost something but not much and if you keep them starved to the edge of death you don’t even need to feed them much. Just replace them with younger, stronger slaves.
Of course, every once in a while, the slaves will produce a Spartacus who decides to burn the whole thing down but that’s what the legions are for anyway. And crucifying a few thousand slaves along the Appian Way is a good reminder of who’s boss.
But even slavery is not the whole story. When the majority of Romans were plebians living on family farms, and these same farmers were the citizen soldiers who were legionnaires when an enemy threatened, there was a patriotism that made the Romans the greatest military force of their age. They might lose twenty or thirty thousand men against Hannibal and then come back and field an army just as large from volunteers who served to protect their nation, their families and their homes.
But the patricians who ran the senate were greedy and selfish and after a soldier returned home from war, he stood to lose his land because he would default on the debts that he couldn’t pay off due to his absence and loss of harvest. These patricians would scoop up these small farms and then use their connections to obtain the defeated enemies as slaves and outcompete the remaining small farmers who couldn’t match the efficiency of the large plantations. And in addition, the patricians eventually discontinued the practice of plebians obtaining conquered lands as colonists. The patricians turned these foreign lands into even larger plantations for themselves.
So eventually these landless plebians ended up as a discontented rabble living on the bread and circuses that the emperors provided in Rome. And eventually they ended up in the armies which was the profession of last resort. So essentially the Roman elites eliminated the middle class. And eventually that reduced the effectiveness of the legions too. And finally, manpower was so depleted through destruction of the actual Roman citizens that the legions more and more were populated with barbarians from the fringes of the empire. So much so that by the fourth and fifth centuries the Franks, Germans, Goths and other pastoral peoples in Gaul and the northern border were the legions.
When Atilla finally invaded the western empire, it was Frankish legionnaires and even generals who resisted him most effectively. But at this point, the jig was up. The barbarians realized that the empire was a paper tiger. Its citizens were downtrodden serfs who couldn’t or maybe wouldn’t take up arms to defend their “nation.” Being ruled by a Roman emperor or an Ostrogothic “king” was a matter of complete indifference to them. In point of fact the barbarian could probably provide better protection and possibly the taxes would be lower if the imperial bureaucracy ceased to exist.
So, whether the Romans could have produced more defensible borders against the nomadic Eurasian pastoral peoples, whether Huns, Alans or even the westernmost tribes like the Goths and Vandals is probably immaterial. If a nation doesn’t have a population willing to defend its homeland, then it is as good as done.
And this was known all the way back in the time of Aristotle. Strong states had a large prosperous middle-class. Because this is the group that produces children and believes in the value of the nation. Not the very rich and not the desperately poor. Rome committed slow suicide when it allowed the patricians to destroy the plebian farmer/soldiers. Because the early Romans so thoroughly defeated the neighboring peoples it took five hundred years to produce new enemies to tear down the rotted corpse of Rome but by the time of Marcus Aurelius the damage had already been done. It just took three hundred more years for Atilla to show up and prove to the Germans that there was nothing there to stop them.
As a footnote I think it’s interesting to see that our modern-day patricians (elites) are busy destroying the working and middle classes and importing migrants to act as cheap (slave) labor to replace them. What a brilliant plan. But not very original.