http://en.wikipedia.org/wiki/Just_war

Just War Theory has two sets of criteria. The first establishing jus ad bellum, the right to go to war; the second establishing jus in bello, right conduct within war.

Jus ad bellum

Just cause

The reason for going to war needs to be just and cannot therefore be solely for recapturing things taken or punishing people who have done wrong; innocent life must be in imminent danger and intervention must be to protect life. A contemporary view of just cause was expressed in 1993 when the US Catholic Conference said: "Force may be used only to correct a grave, public evil, i.e., aggression or massive violation of the basic human rights of whole populations."

Comparative justice

While there may be rights and wrongs on all sides of a conflict, to override the presumption against the use of force, the injustice suffered by one party must significantly outweigh that suffered by the other. Some theorists such as Brian Orend omit this term, seeing it as fertile ground for exploitation by bellicose regimes.

Legitimate authority

Only duly constituted public authorities may wage war.

Right intention

Force may be used only in a truly just cause and solely for that purpose—correcting a suffered wrong is considered a right intention, while material gain or maintaining economies is not.

Probability of success

Arms may not be used in a futile cause or in a case where disproportionate measures are required to achieve success;

Last resort

Force may be used only after all peaceful and viable alternatives have been seriously tried and exhausted or are clearly not practical. It may be clear that the other side is using negotiations as a delaying tactic and will not make meaningful concessions.

Proportionality

The anticipated benefits of waging a war must be proportionate to its expected evils or harms. This principle is also known as the principle of macro-proportionality, so as to distinguish it from the jus in bello principle of proportionality.

A just War is one that avenges wrongs, when a nation or state has to be punished for refusing to make amends for the wrongs inflicted by its subjects or to restore what it has seized unjustly.
In modern terms just war is waged in terms of self defence or in defence of another with sufficient provocation a nation could justify strike first in self defence or defence of an innocent third party. must have the right intention.

Jus in bello

Once war has begun, just war theory also directs how combatants are to act:(Jus in bello)

Distinction

Just war conduct should be governed by the principle of distinction. The acts of war should be directed towards enemy combatants, and not towards non-combatants caught in circumstances they did not create. The prohibited acts include bombing civilian residential areas that include no military target and committing acts of terrorism or reprisal against ordinary civilians.

Proportionality

Just war conduct should be governed by the principle of proportionality. An attack cannot be launched on a military objective in the knowledge that the incidental civilian injuries would be clearly excessive in relation to the anticipated military advantage (principle of proportionality).

Military necessity

Just war conduct should be governed by the principle of minimum force. An attack or action must be intended to help in the military defeat of the enemy, it must be an attack on a military objective, and the harm caused to civilians or civilian property must be proportional and not excessive in relation to the concrete and direct military advantage anticipated. This principle is meant to limit excessive and unnecessary death and destruction.