Rome HAD to defend herself into three continents!

Explanation: Despite Rome's extremely aggressive posture, it actually took a great deal of effort to justify its wars, at least to itself. This seems unremarkable, but the consistency with which it was regarded is somewhat unusually in the ancient world - while rulers like Cyrus the Great and Alexander the Great based their conquests on divine right and the notion that the conquered belonged under their rule, Rome had a very legalistic approach to warfare that demanded just cause.

Of course, Rome was extremely opportunistic and warlike, so what that really amounted to was a little bit of diplomatic snooping for a cause that might be helpful. Rich foe has a weak neighbor? 'Befriend' that neighbor and then use any insult as a pretext to defend a 'friend of Rome'! Roman citizen got punished 'too harshly' for a little bit of innocent crime and grifting? Demand reparations, and then declare war if they aren't provided! Polity violated Clause III Subsection B of their last treaty, according to an extremely legalistic and literal interpretation of the text? Time to loot their cities and sell them into slavery!

The Romans themselves regarded this somewhat seriously - when one of the most powerful (and unambiguously the richest) men in Rome, Crassus, went off to fight Parthia, the suspicion that Parthia, rich and powerful and foreign and threatening though it was, had done nothing wrong was enough to raise the ire of the citizenry, and a prominent citizen went so far as to call down a curse of the gods upon the entire expedition for being an unjust war! It worked, apparently, since Crassus suffered a humiliating defeat and was killed by Parthian troops. Likewise, in Roman Imperial histories, Emperors are routinely praised or condemned for how 'just' their wars seems to be, separate from the success or failure of the venture.