Is this a homework question based on a particular course syllabus or reading? If this is not in reference to specific medieval writers, then it is impossible to answer. Or rather, any and every possible answer you might want to hear can be found among the scribes, notaries, monks, abbesses, bishops, troubadours, clerks, popes, theologians, warriors, mendicants, king, queen, prince, baron, duke, count, inter alia who populate the nearly 1000 years of the medieval period. There was no commonly-held medieval philosophy, rightness/wrongness, justified/unjustified, or any other dualism one wishes to deploy for 'war and peace'.

I recommend you restate the question.