Explanation: Rome, despite its extremely expansionist posture, actually did not (at least nominally) go out a-conquerin' for the sake of conquering. Rome placed a high value on the idea of ius bellum - 'justified war'. Not just "The gods decided we should rule this land!" but something real, like "The enemy raided our lands and we can no longer trust their government."
Of course, 'real' and 'substantial' do not always imply one another - a 'justified' war might come down to something as small as a violation of a single clause of a treaty, or a single Roman citizen being mistreated. After all, if the name of 'Roman citizen' cannot protect a man, it is all-but admitting the impotence of the Republic!
THEY HAD TO CONQUER THE ENTIRE COUNTRY IN RESPONSE IT WAS SELF-DEFENSE