Jus Ad Bellum
How do Just War theories expect to justify the pursuit of some wars? How can we ever conclude that some particular war may be more moral than another? Although there are some differences in the principles used, we can point to five basic ideas which are typical.
These are categorized as jus ad bellum and have to do with whether or not it is just to launch any particular war. There are also two additional criteria which are concerned with the morality of actually waging a war, known as jus in bello, which are covered elsewhere.
The idea that the presumption against the use of violence and war cannot be overcome without the existence of a just cause is perhaps the most basic and important of the principles underlying the Just War tradition. This can be seen in the fact that everyone who calls for a war always proceeds to explain that this war would be pursued in the name of a just and righteous cause--no one ever actually says "our cause is immoral, but we should do it anyway."
The principles of Just Cause and Right Intention are readily confused, but differentiating them is made easier by remembering that the cause of a war encompasses the basic principles behind the conflict. Thus, both "preservation of slavery" and "spread of liberty" are the causes which might be used to justify a conflict--but only the latter would be an example of a Just Cause. Other examples of just causes would include the protection of innocent life, defending human rights, and protecting the ability of future generations to survive. Examples of unjust causes would include personal vendettas, conquest, domination, or genocide.
One of the main problems with this principle is alluded to above: everyone believes that their cause is just, including the people who seem to be pursuing the most unjust causes imaginable. The Nazi regime in Germany can provide many examples of causes which most people today would regard as unjust, but which the Nazis themselves believed were quite just. If judging the morality of a war simply comes down to which side of the front lines a person is standing, just how useful is this principle?
Even if we were to resolve that, there would still be examples of causes which are ambiguous and hence not obviously just or unjust. For example, would the cause of replacing a hated government be just (because that government oppresses its people) or unjust (because it violates many basic principles of international law and invites international anarchy)? What about cases where there are two causes, one just and one unjust? Which is considered dominant?
Principle of Right Intention
One of the more fundamental principles of Just War Theory is the idea that no just war can come out of unjust intentions or methods. For a war to be judged "just," it is necessary that the immediate goals of the conflict and the means by which the cause is achieved be "right"--which is to say, be moral, fair, just, etc. A just war cannot, for example, be the consequence of a desire to greedily seize land and evict its inhabitants.
It is easy to confuse "Just Cause" with "Right Intentions" because both seem to speak about goals or aims, but whereas the former is about the basic principles for which one is fighting, the latter has more to do with the immediate goals and the means by which they are to be achieved.
The difference between the two can be best illustrated by the fact that a Just Cause may be pursued through wrong intentions. For example, a government might launch a war for the just cause of expanding democracy, but the immediate intentions of that war may be to assassinate every world leader who even expresses doubts about democracy. The mere fact that a country is waving a banner of freedom and liberty does not mean that the same country is planning on achieving those goals through fair and reasonable means.
Unfortunately, humans are complex creatures and often perform actions with multiple intersecting intentions. As a result, it is possible for the same action to have more than one intention, not all of which are just. For example, a nation might launch a war against another with the intention of eliminating a dictatorial government (in the cause of expanding liberty), but also with the intention of installing a democratic government which is more favorable to the attacker. Toppling a tyrannical government may be a just cause, but toppling an unfavorable government in order to get one you like is not; which is the controlling factor in evaluating the war?
Principle of Legitimate Authority
According to this principle, a war cannot be just if it has not been authorized by the proper authorities. This may seem to make more sense in a medieval setting where one feudal lord might try to wage war against another without seeking the authorization of the king, but it still has relevance today.
Granted, it is very unlikely that any particular general might try to wage war without some authorization from his superiors, but what we should pay attention to is who those superiors are. A democratically elected government which initiates a war against the wishes of (or simply without consulting) the populace (who, in a democracy, are sovereign like a king is in a monarchy) would be guilty of waging an unjust war.
The main problem with this principle lies in identifying who, if anyone, qualifies as the "legitimate authority." Is it sufficient for a nation's sovereign(s) to approve? Many think not and suggest that a war cannot be just unless it is initiated in accordance with the rules of some international body, like the United Nations. This might tend to prevent nations from going "rogue" and simply doing whatever they want, but it would also constrain the sovereignty of the nations who abide by those rules.
In the United States, it is possible to ignore the UN question and still be faced with a problem of identifying the legitimate authority: Congress or the President? The Constitution gives Congress the exclusive power to declare war, but for a long time now presidents have engaged in armed conflicts which have been wars in all but name. Were those unjust wars because of that?
Principle of Last Resort
The principle of "Last Resort" is the relatively uncontroversial idea that war is awful enough that it should never be the first or even the primary option when it comes to resolving international disagreements. Although it may at times be a necessary option, it should only be chosen when all other options (generally diplomatic and economic) have been exhausted. Once you have tried everything else, then it is presumably more difficult to criticize you for relying upon violence.
Obviously, this is a condition which is difficult to judge as having been fulfilled. To a certain degree, it is always possible to try one more round of negotiations or impose one more sanction, thus avoiding war. Because of this war may never truly be a "final option," but the other options may simply not be reasonable--and how do we decide when it is no longer reasonable to try to negotiate more? Pacifists can argue that diplomacy is always reasonable while war never is, suggesting that this principle is neither as helpful nor as uncontroversial as it first appeared.
Practically speaking, "last resort" tends to mean something like "it's not reasonable to keep trying other options"--but of course, what qualifies as "reasonable" will differ from person to person. Although there can be broad agreement on it, there will still be honest disagreement on whether we should keep trying non-military options.
Another interesting question is the status of pre-emptive strikes. On the surface, it seems as though any plan to attack another first cannot possibly be the last resort. However, if you know that another country is planning to attack yours and you have exhausted all other means to convince them to take a different course, isn't a pre-emptive strike actually your final option now?
Principle of the Probability of Success
According to this principle, it isn't "just" to launch a war if there is no reasonable expectation that the war will be successful. Thus, whether you are faced with defending against another's attack or considering an attack of your own, you must only do so if your plans indicate that victory is reasonably possible.
In many ways this is a fair criterion for judging the morality of warfare; after all, if there is no chance of success, then many people will die for no good reason, and such a waste of life cannot be moral, can it? The problem here lies in the fact that a failure to achieve military objectives doe not necessarily mean that people are dying for no good reason.
For example, this principle suggests that when a country is attacked by an overwhelming force which they cannot defeat, then their military should submit and not try to mount a defense, thus saving many lives. On the other hand, it can plausibly be argued that a heroic, if futile, defense would inspire future generations to keep up a resistance to the invaders, thus eventually leading to the liberation of all. This is a reasonable objective, and although a hopeless defense may not achieve it, it doesn't seem fair to therefore label that defense as unjust.