It's way too long to quote here, but try Christine de Pizan's The Book of Deeds of Arms and of Chivalry (Penn State Press, 1999) discussion of war -- Part I, chapter II ff. in my copy (Sumner Willard translation -- I can't read 15th Century French!). Even in 1410 a distinction was made between fighting in defense of yourself and your country and wars of aggression.