The poverty that made that possible was due to the three dozen or so large families who owned most of Italy; when crises erupted (such as an invasion) the common soldiers were patriotic volunteers; when they got back after a year, or two, or more, of a successful campaign, they’d find their lands had been confiscated by some rich a-hole — sort of like eminent domain is used now, but less subtle than that. So, the impoverished classes grew, along with the percentage of the population who’d been enslaved. The insatiable demand for more slaves is another thing that made Caesar’s conquest of Gaul so popular; the growth in slave labor also led to greater poverty among those unable to find work for themselves.
PS It is nice to meet fellow Roman history buffs here in the Free Republic.