The Roman Empire got large because of the "military" complex that the society had at the time. In order to advance socially (status, and wealth) being a milita

The Roman Empire got large because of the "military" complex that the society had at the time. In order to advance socially (status, and wealth) being a military commander was a must. If you had victories in your belt you had more status within the society. Just being rich, didn't give you more status than somebody that was successful in combat, but it gave you the ability to fund wars and expeditions in order to gain more riches. Hence, since the personal ambitions of important people in Rome fueled the military complex and all the invasions that came with it. Most wars were justified as need for the the protection of Rome. Even full scale invasion and slaughtering of other tribes (either in the North with Germanic tribes, or South East with Parthia). This all had an origin when Rome was born and had to fight its neighbor italic tribes to get where it was. Then it was the large Punic wars with Carthage that truly cemented the military complex in the society. It was a brutal war (half of soldiers that fell for the Roman Empire over centuries, died during the Punic wars), that threaten the existence of Rome itself. So, some of the military complex was justified historically (either be strong, or get subjected), it was hijacked and amplified by people's personal ambitions. (either riches, becoming emperors, etc).... Similarly, today the military complex in the US was born for needed reasons (WW1 and WW2), but it is so large that it needs reasons to exist, and create wars when not needed (second Iraqi war as the worst example of this).