The decay of the Roman free peasantry who also served in the pre-Marian army as the archetypal citizen-soldier. The welfare-dependent Roman "mob" and the political necessities and opportunities of pandering to it were all ultimately the product of consolidation of farmland into immense estates owned by the patrician elite and worked by slaves. I don't know to what degree freeborn urban tradesmen and laborers also suffered from slave competition, but it would only have added to the problem if they did.
The Marian reforms transformed the Roman army from a part-time and essentially conscripted combined arms force - everything from light infantry to heavy cavalry plus "artillery" armed with slings, javelins, and bow and arrows: all ultimately determined by how much money you could spend on equipment and training - that campaigned during the fallow season and then returned to its farms ... to a professional standing army based heavily on the archetypal legionary with short sword and shield - backed up by mechanized artillery - who was equipped and trained at the state's expense and dependent for his livelihood upon the Republic's and then the Empire's demand for fighting men and then upon the loot and slaves extracted from a successful conquest.
Ancient Greece used organized, state-sponsored colonization to both unburden the
polis
and expand its circle of trading partners and military allies by shipping off its landless freemen - whom the ideal Greek socio-economic order had no place for - and setting them up as landowning citizens and hoplites of a new daughter
polis
. But to my knowledge, Rome had no equivalent program other than the land grants to legionaries (after 20 years of service! And worked by whom? Surely not by a 40+ year old man nor by the sons he didn't have while in the legion.): grants that were discontinued early in Rome's expansionist period in favor of cash payment.
The decay of the ever-praised Roman virtue - or rather
virtus
- while undeniable, can probably be traced back to these less glamorous but more profound structural developments, since they dissolved the organic basis of that virtue. Baths and games didn't do it on their own; not even close. Patrician conservatives during the late Republic and early Empire were left preaching the fossilized values of a society that had ceased to exist a century or more ago, which even their own class only play-acted at their villas ... like Marie Antoinette on her toy dairy farm.