Well, this hasn't really stopped anyone, has it? The international law has been nothing but a device or rather a pretext to validate political actions. Political actions of the strong. That being sad war has more of a human and natural element to it.
The modern day legitimation of war arises from defending or preserving territory, human life or beliefs. Even when a state actor or an organization is belligerent, they always have an excuse to wage war. As fake as this legitimation is, this is the green light to engage in murder - among other things. So when we have the "right" to kill, why do we need laws (i. e. the Geneva Conventions) that specify how to wage war? Why are wounded persons or prisoners of war exempt from lethal actions? This is merely a humane and civilized paint job on the gruesome reality called war.
Everyone who has watched Discovery Channel or some kind of documentary knows how cruel nature is. As the strongest mammals patrolling the Earth, we only are a slight exception to the law of nature: The strong prevails. Humans are very well capable of taking a life for one's own benefit, mating with a partner against their will or stealing objects for one's own enrichment and perhaps for survival. While we're capable of these things, we refrain from them, because there's the Leviathan[2] who would punish us if we commit these acts. Usually. Even in civilized societies, we see individuals who cannot resist their hard-wired, natural urges or their risk assessment concludes that such grievances will go either unpunished or the reward outweighs the castigation.
On the battlefield we do not have the Leviathan to enforce the laws. Even if your superior would enforce such laws, he's not necessarily strong enough to uphold them. Another question that has to be answered is: Who exactly gives you the authority to enact laws or apply laws on foreign soil? When the U.S. invades Iraq, they are on Iraqi soil and not on U.S. territory. Therefore, even if U.S. soldiers commit "war crimes", they should be judged by Iraqi laws. The only reason why they are not judged so is because the U.S. is more powerful than Iraq.
The international laws imposed on soldiers however do not account for the human factor. When a soldier snaps and commits "atrocities," is he actually culpable of his actions? The jurisprudence does take several factors in account. For example, did these actions occur while the aggressor was under the influence of drugs? On one's homeland, such circumstances are considered as mitigating. But what about the tremendous stress a soldier has to shoulder in the war zone? Does being put into such an environment excuse any misconduct? This is merely an intellectual discussion divorced from reality. For example, it's expected by international laws and social norms to treat the prisoners of war well. The exact same people who have been shooting at you with the intention of killing you a mere minute before. Or they even have killed your buddy and you are burning with vengeance. In a civilized society, we have given our right to exact retribution to the Leviathan, to the judicial system.
There's no such institution on the battlefield. That means that enemy soldiers literally get away with murder. In this case we have two problems. First, the need of justice that each individual objectively perceives is not satisfied because there's no retribution and second, the act of exercising free will in such a manner that would benefit the soldiers who have captured the enemy is lost. Because executing the enemy (reducing the number of your opponents, preventing them to rejoin the war afterward), torturing them (for information - as ineffective as it may be - the effect of catharsis, satisfying one's need for revenge) or raping them (for pleasure, for degradation and demoralization, causing long-lasting trauma to make them ineffective on the battlefield or making them dysfunctional in later stages of life, so the threat they could pose in the future is considerably diminished) do benefit your faction.
This shows why such laws, as difficult to enforce as they may be, impede countries from waging war effectively. By adhering to such moral laws - for that's what they are - we give up our potential to effectively engage in combat. This however doesn't mean that we should engage in so called "war crimes" for the sake of it. It's tremendously valuable to have a good relationship with the locals to stop insurgencies. The locals will less likely help you, if you rape their women or burn their villages for shit and giggles.
On my previous post I briefly talked about our inability to wage unrestricted warfare. Not only is it frowned upon to make territorial gains in the post World War II era where "adjusting" the world map is anathema (barring a few irrelevant states), but the moral compulsion of the world population is preventing states from going through with it. The so-called "human rights," purported by the "morally just," who have no stake in the game (i. e. intellectuals and politicians commenting from safety - safety, that's assured by a strong military or economic power - except feeling validated have hindered countries, in other words entities that are bound by some kind of law codex to fight effectively. On a side note, it's quite interesting that public opinion is holding so much power. Much more than physical actions.
It's not that we lack the instruments to win wars, it's just that the society that has grown up in peace and cannot fathom war on their doorsteps is blocking their countries from waging wars. States like China have been using softer measures to accomplish their mission in conquering minorities. It's possible to destroy your enemy by erasing their cultural identities and marrying their women. While this approach generates less heat, it is also a fairly slow process. Nevertheless it's the process that you apply after conquering territories to prevent any uprisings.