How the Crusade Became a Medieval Institution - Medievalists.net
The institution of the crusade was constructed from the cultural narrative of Christian 'holy war' (bellum sacrum). This included the idea of warfare as a religious act.
How Medieval Thinkers Justified War: From Augustine to Aquinas - Medievalists.net
Augustine understood war as a consequence of humanity's fallen nature, viewing it as a more common state of affairs than peace, which he considered merely an interlude.