Broadly speaking there are three prescriptive theories of human warfare: Ideal Pacifism (Lethal aggression by nations (or other political entities) is never morally justified.) Political Realism (Lethal aggression by nations is justified based solely on utility), and Just War Theory (Lethal aggression by nations is sometimes morally justified.)
All three theories are deeply problematic. The basic problem with Ideal Pacifism is that, if the use of lethal force is never morally justified, then self-defense (or the defense of the weak) must be limited to non-lethal means; which rarely works against lethal aggressors. Critics argue that: "If life is of infinite value, then why is it not worth defending?" In fact, lethal aggressors prefer to wage war against pacifists (especially wealthy ones!)! After all, passivist nations are easy targets for nations willing and able to employ lethal force. The problem with Political Realism is that if every nation on earth goes to war whenever it's leaders judge that the benefits outweigh the costs, the incidence and lethality of war would increase substantially. And (as we will see below) the basic problem with Just War Theory is that the criteria are so vague and malleable, that almost any war or weapon can be justified.
As I write, President Obama is asking Congress to vote on whether to bomb Syria in response to its (apparent) use of chemical weapons against it's own citizens. Any arguments over U.S. intervention in Syria entails the application of the principles of Jus ad Bellum and Jus in Bello. Let's look at both sets of criteria.
Jus ad Bellum: Principles for Engaging in War
C1: There must be a just cause for entering war. The only just cause for entering war is defense from lethal aggression: "self-defense" or to "defend the weak." You cannot go to war in order to secure resources or to punish lethal aggressors.
C2: Only a proper authority can declare war. Generally, this refers to the "legitimate political leader(s) of a nation." One problem here is that during times of civil war, there may be no "proper authority." In the U.S. multiple "authorities" have the authority to declare war. In the past, the President declared war and Congress decided whether or not to pay for it. Since Vietnam, Congress has not been a responsible steward of U.S. resources.
C3: The decision to declare war must be accompanied by the right intention. This means that legitimate leaders must really "intend" to end lethal aggression, and NOT intend to secure resources or punish lethal aggressors. Here it is not always easy to determine "intent," as many leaders lie over their true intent.
C4: Going to war must be the last resort: all non-lethal means must be exhausted first. The usual non-lethal means include: passive resistence such as protests and economic boycotts. The problems here are that "non-lethal means" rarely work, and it's difficult to decide how long to employ these non-lethal means.
C5: The benefits of engaging in war must outweigh the costs. Combatants must determine whether the amount of pillage, plunder, and death required to end a war is worth it. Here the problem is that wars are so complicated that it's impossible to generate accurate cost/benefit analyses. Wars are rife with unanticipated consequences. President Bush predicted that the war in Iraq would last a few months.
C6: There must be a reasonable chance of success. This means that combatants may not engage in wars of futility. Combatants must decide beforehand what ultimate "success" means within any given context. If a war is hopeless, then it cannot be justified. Again, before the 1960s, very few war critics argued that the war in Vietnam was futile. Some scholars still argue that it was winnable, but poorly executed.
Jus in Bello: "Principles for Conducting War"
C7: Combatants must exercise discrimination; that is combatants may only deliberately target enemy combatants and not non-combatants. This usually means that women and children and residential areas may not be targeted. Killing non-combatants is, however, morally acceptable if they are killed accidentally, as an unintended "side-effect" of targeting legitimate combatants.
C8: Combatants must exercise proportionality in the use of lethal force. They may not use any more lethal force than is necessary to end the war. It is never acceptable to kill all enemy combatants and/or destroy all property.
C9: Combatants may not employ any evil means or war strategies that are inherently immoral. This usually refers to the use of "weapons of mass destruction" or weapons that inflict an extraordinary amount of pain and suffering, such as chemical weapons or biological weapons. Rape, pillage, theft, and torture are generally considered to be a priori immoral; even if they "work."
C10: Combatants must exercise benevolent quarantine when it captures enemy combatants. This generally means that prisoners of war must be adequately fed and clothed and provided adequate health care. Torture of prisoners is never justified.
C11: When a war is over any violators of the principles of just war must be held responsible. This usually entails war trials. This usually means that the winners of the war take the losers to court, execute war criminals and impose financial costs on the losers. And of course, powerful nations, whether they win a war or not are never tried for war crimes.
In summary, most of the world still refer to Just War Theory as the basis for arguing over the basis for going "to war" and arguing over what can be done "in war." How would you analyze the justice of the following U.S. wars: War in Iraq, War in Afghanistan, and the looming in Syria. How would you analyze the justice of following weapons: atomic bombs, torture, rape, chemical and biological weapons, and armed drones to kill terrorist leaders.