In college, a history professor said they did not -- maybe (not sure if I recall correctly) that ideas like that are modern and Rome wanted the slaves and treasure and so they just went ahead.
EDIT: Seems clear enough but what I meant was, Did Rome suggest that they were doing it for national security or to spread civilization or to prevent the domino-like expansion of the Gauls or whatever?
Yes, they did. They argued that their conquests bring benefits of peace, civilization and prosperity to the conquered lands. This point of view was expressed not only by native Roman writers (like Cicero) but also by some writers from the conquered nations (Polybius, for example).
In the case of Greece, they certainly did not bring civilization to Greece, just the opposite happened. But they brought peace. The whole history of ancient Greece is the history of continuous cruel warfare between the city-states. And all attempts to stop it and unite the Greek states before the Roman conquest failed. Writers of that time perfectly understood that "Greek freedom is essentially the freedom to make war on each other", I don't remember which (Greek) writer said that.
The same argument was used by some British empire builders (The "burden of white man" as Kipling called it). Some modern scientists agree with this argument, for example:
Ian Morris, War! What is it good for?: Conflict and the Progress of Civilization from Primates to Robots, 2014.
EDIT. I do not say anything about the plausibility of this argument, but this was not asked. In some cases indeed it looks like Roman conquest brought peace (Greece). In other cases it led to complete destruction of the conquered society (Carthage).
