At the same time, Roman society was a highly stratified class system whose divisions were bubbling below the surface. This system consisted of noble families of the senatorial rank, the knight or equestrian class, citizens (grouped into two or three classes depending on the time period - self-governing allies of Rome, landowners, and plebs or tenant freemen), non-citizens who lived outside of southwestern Italy, and at the bottom, slaves. By law, only men who were citizens could vote in certain assemblies, and only those men who owned a certain amount of real property could serve in the military, which would gain them social prestige and additional benefits of citizenship. The government owned large tracts of farmland ("ager publicus") that it had gained through conquest or escheat (acquisition from owners who had died without heirs); this it rented out to large landholders who used their slaves to till it or who sub-leased it to small tenant farmers. There was some social mobility and limited suffrage. The plebs (or plebeians) were a socio-economic class, but also had possible origins as an ethnic group with its own cult to the goddess Ceres, and ultimately, were a political party during much of the Roman Republic. This social system had been stable after the Conflict of the Orders, since economically both the patricians and the plebeians were relatively both well off. Italy was dominated by small landowners. However, sometime after the Punic Wars, this changed due to various factors. Partly due to the availability of cheap grain coming into the Roman food supply, as well as the social displacement caused to farmers who had to serve on long foreign campaigns using their own financial resources and often having to sell out, the countryside came to be dominated by large estates ("latifundia") owned by the Senatorial order. This led to a population explosion in Rome itself, with the plebeians clinging desperately to survival while the patricians lived in splendor. This income inequality severely threatened the constitutional arrangements of the Republic, since all soldiers had to be property owners, and gradually property owning was being limited to a small Senate, rather than being evenly distributed across the Roman population.