Jus ad bellum

Legitimate authority.  Private individuals and groups are not permitted to take up arms against others, however justified their cause may appear. Only legitimate governments, i.e., those who have been entrusted with the common good, may wage war, and they must do it openly and with rightful authority.

Just cause.  A government may wage war in self-defense, in defense of another nation, to protect innocents, to regain something wrongfully taken, or to punish those who have done evil. The desire for personal gain or revenge, or to impose unlawful rule, is never an acceptable cause for waging war.

Right intention.  The establishment of peace must be the ultimate goal of a sovereign power in waging war, and so it may not use war as a pretext for its own selfish goals.

Last resort.  The legitimate public authorities must reasonably exhaust all other diplomatic and non-military options for securing peace before resorting to war.

Reasonable chance of success.  The governing authorities may not resort to war unless its prospects for success are good. This will protect the against the loss of innocent life in the pursuit of a hopeless cause.

Proportionality.  A sovereign power must respond to aggression or evil with military force only when the effects of its defensive actions do not exceed the damage done by the aggression or evil itself.

Jus in bello

Noncombatant immunity.  An authority waging war is morally obligated to seek to discriminate between combatants and noncombatants. While civilians unfortunately may sometimes come into harm's way, a government may not deliberately target them.

Proportionate means.  This principle concerns the specific tactics of warfare and seeks to restrict unnecessary use of force. It is intended to ensure that the military means used to achieve certain goals and goods are commensurate with their value, particularly when compared to the loss of life and destruction that could also occur.