The Romans achieved so many conquests because of how they dealed with areas newly conquered. They did not colonize them; instead they expanded Roman citizenship. When a place was conquered, it became Roman. This strategy of absorption instead of colonization strenghtened their army and held the empire together.
When the Romans conquered the entire Mediterranean, calling it "Mare nostrum" (our sea), they faced the consequences of the expansion.
I) Slavery and land concentration increased immensely.
II) Retired legionaires wanted more land so more conquests had to be made.
III) Fighters in the wars couldn't cultivate their land, so they lost it to rich men. This caused an exodus from the cities.
IV) In the long term this would led to the decline of the Roman Empire and to the process of feudalization of Europe.
