Before Gaius Marius, legions were called up largely through conscription. This worked well as long as a) there were plenty of strapping young lads growing up on family farms who could take up the family armor and march off to war; and b) campaigns were strictly local affairs and the men could be back home in time to help out with the harvest.
But as Rome became the dominant force in Italy, its wars became lengthier matters requiring multi-year campaigns. The yeoman farmers, absent for years on end, returned to find their farms bankfupt and bought out by wealthy landowers. The farmland of Italy was reorganized into giant plantations (latifundiae), worked by slave labor, that pulled a Wal-Mart on the remaining family farmers and drove them out of business. The cities, particularly Rome, played host to ever-larger numbers of indigent citizens who lived hand-to-mouth, and became dependant on the periodic grain doles issued by the Senate (can we say “welfare state?”).
Then one fateful day (around 133 B.C.) Tiberius Gracchus took a ride through the farming countryside outside of Rome. Distressed to find mile after mile of slave-worked latifundia and not a single family-owned farm, he sought election as tribune of the plebs and set about trying to reverse this trend. His agrarian reform bills would sow the first seeds of the hundred year struggle known as the Roman Revolution.
One aspect of this struggle dealt with the nature and composition of Rome’s armies. The old school conservatives believed that the only soldiers worth their salt (sorry) were those who owned property and were therefore considered to have a genuine stake in affairs of the state. But as Rome’s armies went off to one campaign after another, frequently led by incompetent commanders, the number of propertied citizens who qualified for conscription duty sunk to levels completely incapable of sustaining the need for manpower.
It took the rise of Gaius Marius (the brother-in-law of Julius Caesar’s father) to shake things up. Over the strident objections of the senate (whose minions had assassinated Tiberius Gracchus and his brother Gaius), Marius legislated a the opening of the legions to those who had previously been ineligible to serve due to poverty.
While, on the one hand, this move certainly opened a untapped reservoir of manpower for the military, it also led eventually to the development of armies that were more loyal to their commanders than to the State. The feud between Marius and his protégée Sulla eventually led to civil war as each commander brought his armies into the fray.
From then on, a professional army became the norm.