How did rome grow from a single city to the center of a huge empire?

1. The values of the Roman republic, including rule of law, the rights of citizens, upright moral behavior, and keeping one's word 
2.Poor soldiers hoped for land, loot, or salaries.
The well-to-do or well-connected gained great estates, earned promotion, and sometimes achieved public acclaim and high political office by participating in empire building.
3.Expansion was necessary to supply Rome with the slaves necessary to keep labor supplies high enough for Rome to function effectivly.