A declaration by the ruler to defend the "common good" against enemies.

A "just cause" for an attack on an enemy "because they deserve it on account of some fault" such as avenging wrongs they have committed.

A "rightful intention" to advance good or avoid evil such as punishing evil-doers and not simply grabbing land or goods. 

A “just morality” in that that belligerents disagree on moral platitudes.