Gaius Marius was the first on to go to the capite censi (the headcount), the poor who could not afford to supply their own equipment (a requirement for the legions up to this time). He basically said "Join my legion, I'll pay the expenses and give you a wage". The army then became a profession.
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.