How did the government of rome develop into an empire

Rome was initially a city-state with about 50 square miles of territory to sustain itself. With small farms, the citizens could not split them up between their sons and so sought extra land for them. The surrounding city-states were bent on the same need, so they clashed, with Romme losing some, but on average winning most fights. As winners they took land and expanded. 

This expansion created more strife, and expansion, leading to dominance of Italy. As a rising power, cities in southern Gaul (France), Spain and Sicily sought Rome's assistance, and as a land power it allied itself with Carthage, a sea power. This arrangement came to an end when Rome supported Greek cities in Sicily against Carthaginian encroachment. A win in the First Punic (Carthaginian) War put the Romans on the path to empire, which was consilidated with the extirmination of Carthage in the Third Punic War in 146 BCE. At this stage Rome dominated the Western Mediterranean.