That war may be just the following conditions must be fulfilled: It must be declared by the State itself; it must be necessary in the last resort after diplomacy has failed; there must be a grave and just reason for it; the method of it must be just and in accordance with international law; an upright purpose must be intended; it may not be protracted after due satisfaction has been given or offered; the conditions of peace must be just, and may not be crushing, unless such severity is necessary for present self-defence.

When the reasons for undertaking war are not certainly just, it is more generally taught that war may not be undertaken, for another State may not be deprived of rights in possession, one of these being immunity from attack. But when a State is on the defensive, it is sufficient justification for defence that its own injustice is not obvious.’

It seems to me fair to suggest that the Just War doctrine arose precisely because Christians felt almost instinctively that war needed to be ‘justified'.