The Romans are known as a warliking agressive people. That they certainly were, but not more than the peoples surrounding them. So they had to be, in order not to be destroyed themselves. They needed 300 years to make their first real conquest and after 400 years the city of Rome was conquered and almost destroyed.
To understand fully how they were capable of building their huge empire, it is necesary to understand their culture. By reading the ancient manuscripts, you can learn about their culture bit by bit. Here you are only a mouseclick away from learning about the Roman culture.
Keep in mind that a political carreer was not possible without a military carreer, though one didn't need to excell in both. P.Cornelius Scipio who was victorious over Hannibal was a lousy politician. Cicero a brilliant statesman was no good as a military commander. So you need to explore the Roman army as well as the Roman society where you will find numerous articles about Roman government, Roman religion and Roman private life.