“Generally stated, a just war must be waged 1) by a legitimate government, 2) for a legitimate cause, 3) with a legitimate purpose, 4) with consideration of benefits and costs, 5) with means proportionate to the offense, and 6) recognizing the difference between civilians and soldiers.” -J. Douma, The Ten Commandments
BTW NOTE: these seem pretty subjective, hence the difficulties in defining a ‘just war.’