Why pursuing peaceful policy makes sense (among other foreign policies) even if one lives of the war:
Peace as a tool of conquest
“To keep great princes permanently weak, the Romans did not want them to make any alliance with those to whom they had accorded their own. And since they did not refuse their own to a powerful prince’s neighbors, this condition, stipulated in a peace treaty, left him without allies.
Moreover, when they had conquered some eminent prince, they wrote into the treaty that he could not have recourse to war to settle his differences with allies of the Romans (that is, usually with all his neighbors), but that he would have to use arbitration. This removed his military power for the future.
And, to reserve all such power to themselves, they deprived even their allies of it. As soon as the allies had the least dispute, the Romans sent ambassadors who forced them to make peace. We need only observe how they terminated the wars of Attalus and Prusias.
When some prince had made a conquest, which often left him exhausted, a Roman ambassador immediately arrived to snatch it from his hands. From among a thousand examples, we can recall how, with a word, they drove Antiochus out of Egypt.
When they saw two peoples at war, even though they had no alliance or dispute with one or the other, they never failed to appear on the scene. And like our knights-errant, they took the part of the weaker. Dionysius of Halicarnassus says it was an old practice of the Romans always to extend their help to whomever came to implore it.”
Montesquieu, Considerations on the Causes of the Greatness of the Romans and their Decline, 1734 (CHAPTER VI. THE CONDUCT THE ROMANS PURSUED TO SUBJUGATE ALL PEOPLES)