This is a very difficult question to answer because the Romans were different as far as crossing the line towards conquering. In its early days I think Rome could be looked at as consolodating its hold on central Italy, which can be viewed as trying to protect itself. It established Latin Allies who eventually revolted, several times, causeing Rome to act and eventually become their masters. With the Second Punic War, they were attacked by Carthage and fought back and defeated them, so gained territory in Spain and Africa, same goes for the 4 or so wars with Macedon, in which they were attacked, or suspected of attack, so gained territory there. In the last 100 years of the republic, it began to change a little, especially with the greed of Crassus and the questioned motives of Caesar in Gaul. There were still wars to protect themselves, such as the war with Pontus fought by Sulla, and the affairs of Caesar in Egypt and later Augustus, Caesar felt angry about Pompey the great being killed by Egyptian leader. In the empire the motives were more for conquering than for protection. Some one who has some expertise on the empire can help you out here as my expertise really ends in 44 BCE.