[PapaBear3625]

The Roman welfare state was off the charts. A citizen of Rome had everything for free. Eventually, the freed slaves and African immigrants were the majority and had no dog in the fight. Rome bred itself out, gave away assets to foreign citizens and collapsed from demographics.

As with other societies, Rome developed a hereditary wealthy class. This class imported cheap workers (slaves) to provide them with servants and cheap labor to produce their stuff.

Meanwhile, the Roman middle class, the class from which their armies were recruited and which served to prop up Rome, disappeared or went on the dole.

Finally, they had no people willing to serve in their military out of patriotism, only for cash and benefits. And a mercenary soldier is not generally willing to take risks to get the mission accomplished.

[morphing libertarian]

even when there was patriotism the legionaires wanted land and other spoils. Hard to show an age of pure patriotism making thousands rush into the army.