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The law of war is the component of international law that regulates the conditions for initiating war (jus ad bellum) and the conduct of warring parties (jus in bello). Laws of war define sovereignty and nationhood, states and territories, occupation, and other critical terms of law.
Among other issues, modern laws of war address the declarations of war, acceptance of surrender and the treatment of prisoners of war; military necessity, along with distinction and proportionality; and the prohibition of certain weapons that may cause unnecessary suffering.
The law of war is considered distinct from other bodies of law—such as the domestic law of a particular belligerent to a conflict—which may provide additional legal limits to the conduct or justification of war.
The first traces of a law of war come from the Babylonians. It is the Code of Hammurabi, king of Babylon, which, 2000 B.C., explains its laws imposing a code of conduct in the event of war:
I prescribe these laws so that the strong do not oppress the weak.
In ancient India, the Mahabharata and the texts of Manou's law urged mercy on unarmed or wounded enemies. The Bible and the Qur'an also contain rules of respect for the adversary. It is always a matter of establishing rules that protect civilians and the defeated.
Attempts to define and regulate the conduct of individuals, nations, and other agents in war and to mitigate the worst effects of war have a long history. The earliest known instances are found in the Mahabharata and the Old Testament (Torah).
In the Indian subcontinent, the Mahabharata describes a discussion between ruling brothers concerning what constitutes acceptable behavior on a battlefield, an early example of the rule of proportionality:
One should not attack chariots with cavalry; chariot warriors should attack chariots. One should not assail someone in distress, neither to scare him nor to defeat him ... War should be waged for the sake of conquest; one should not be enraged toward an enemy who is not trying to kill him.
An example from the Book of Deuteronomy 20:19–20 limits the amount of environmental damage, allowing only the cutting down of non-fruitful trees for use in the siege operation, while fruitful trees should be preserved for use as a food source:
19When you besiege a city for a long time, making war against it in order to take it, you shall not destroy its trees by wielding an axe against them. You may eat from them, but you shall not cut them down. Are the trees in the field human, that they should be besieged by you? 20Only the trees that you know are not trees for food you may destroy and cut down, that you may build siegeworks against the city that makes war with you, until it falls.
Also, Deuteronomy 20:10–12 requires the Israelites to make an offer of conditioned peace to the opposing party before laying siege to their city, taking the population as servants and forced-laborers instead, shall they accept the offer.
10 When you draw near to a city to fight against it, offer terms of peace to it. 11And if it responds to you peaceably and it opens to you, then all the people who are found in it shall do forced labour for you and shall serve you. 12 But if it makes no peace with you, but makes war against you, then you shall besiege it.
Similarly, Deuteronomy 21:10–14 requires that female captives who were forced to marry the victors of a war, then not desired anymore, be let go wherever they want, and requires them not to be treated as slaves nor be sold for money:
10When you go out to war against your enemies, and the Lord your God gives them into your hand and you take them captive, 11 and you see among the captives a beautiful woman, and you desire to take her to be your wife, 12 and you bring her home to your house, she shall shave her head and pare her nails. After that you may go in to her and be her husband, and she shall be your wife. 14 But if you no longer delight in her, you shall let her go where she wants. But you shall not sell her for money, nor shall you treat her as a slave, since you have humiliated her."
In the early 7th century, the first Sunni Muslim caliph, Abu Bakr, whilst instructing his Muslim army, laid down rules against the mutilation of corpses, killing children, females and the elderly. He also laid down rules against environmental harm to trees and slaying of the enemy's animals:
Stop, O people, that I may give you ten rules for your guidance in the battlefield. Do not commit treachery or deviate from the right path. You must not mutilate dead bodies. Neither kill a child, nor a woman, nor an aged man. Bring no harm to the trees, nor burn them with fire, especially those which are fruitful. Slay not any of the enemy's flock, save for your food. You are likely to pass by people who have devoted their lives to monastic services; leave them alone.
Furthermore, Sura Al-Baqara 2:190–193 of the Quran requires that in combat Muslims are only allowed to strike back in self-defense against those who strike against them, but, on the other hand, once the enemies cease to attack, Muslims are then commanded to stop attacking:
And fight with them until there is no persecution, and religion should be only for Allah, but if they desist, then there should be no hostility except against the oppressors.
In the history of the early Christian church, many Christian writers considered that Christians could not be soldiers or fight wars. Augustine of Hippo contradicted this and wrote about 'just war' doctrine, in which he explained the circumstances when war could or could not be morally justified.
In 697, Adomnan of Iona gathered Kings and church leaders from around Ireland and Scotland to Birr, where he gave them the 'Law of the Innocents', which banned killing women and children in war, and the destruction of churches.
In medieval Europe, the Roman Catholic Church also began promulgating teachings on just war, reflected to some extent in movements such as the Peace and Truce of God. The impulse to restrict the extent of warfare, and especially protect the lives and property of non-combatants continued with Hugo Grotius and his attempts to write laws of war.
One of the grievances enumerated in the American Declaration of Independence was that King George III "has endeavoured to bring on the inhabitants of our frontiers the merciless Indian Savages whose known rule of warfare is an undistinguished destruction of all ages, sexes and conditions".
The modern law of war is made up from three principal sources:
The opposite of positive laws of war is customary laws of war, many of which were explored at the Nuremberg War Trials. These laws define both the permissive rights of states as well as prohibitions on their conduct when dealing with irregular forces and non-signatories.
The Treaty of Armistice and Regularization of War signed on November 25 and 26, 1820 between the president of the Republic of Colombia, Simón Bolívar and the Chief of the Military Forces of the Spanish Kingdom, Pablo Morillo, is the precursor of the International Humanitarian Law. The Treaty of Guadalupe Hidalgo, signed and ratified by the United States and Mexico in 1848, articulates rules for any future wars, including protection of civilians and treatment of prisoners of war. The Lieber Code, promulgated by the Union during the American Civil War, was critical in the development of the laws of land warfare. Historian Geoffrey Best called the period from 1856 to 1909 the law of war's "epoch of highest repute." The defining aspect of this period was the establishment, by states, of a positive legal or legislative foundation (i.e., written) superseding a regime based primarily on religion, chivalry, and customs. It is during this "modern" era that the international conference became the forum for debate and agreement between states and the "multilateral treaty" served as the positive mechanism for codification.
In addition, the Nuremberg War Trial judgment on "The Law Relating to War Crimes and Crimes Against Humanity" held, under the guidelines Nuremberg Principles, that treaties like the Hague Convention of 1907, having been widely accepted by "all civilised nations" for about half a century, were by then part of the customary laws of war and binding on all parties whether the party was a signatory to the specific treaty or not.
Interpretations of international humanitarian law change over time and this also affects the laws of war. For example, Carla Del Ponte, the chief prosecutor for the International Criminal Tribunal for the former Yugoslavia pointed out in 2001 that although there is no specific treaty ban on the use of depleted uranium projectiles, there is a developing scientific debate and concern expressed regarding the effect of the use of such projectiles and it is possible that, in future, there may be a consensus view in international legal circles that use of such projectiles violates general principles of the law applicable to use of weapons in armed conflict. This is because in the future it may be the consensus view that depleted uranium projectiles breach one or more of the following treaties: The Universal Declaration of Human Rights; the Charter of the United Nations; the Genocide Convention; the United Nations Convention Against Torture; the Geneva Conventions including Protocol I; the Convention on Conventional Weapons of 1980; the Chemical Weapons Convention; and the Convention on the Physical Protection of Nuclear Material.
It has often been commented that creating laws for something as inherently lawless as war seems like a lesson in absurdity. But based on the adherence to what amounted to customary international law by warring parties through the ages, it was believed[by whom?] that codifying laws of war would be beneficial.
Some of the central principles underlying laws of war are:
To this end, laws of war are intended to mitigate the hardships of war by:
The idea that there is a right to war concerns, on the one hand, the jus ad bellum, the right to make war or to enter war, assuming a motive such as to defend oneself from a threat or danger, presupposes a declaration of war that warns the adversary: war is a loyal act, and on the other hand, jus in bello, the law of war, the way of making war, which involves behaving as soldiers invested with a mission for which all violence is not allowed. In any case, the very idea of a right to war is based on an idea of war that can be defined as an armed conflict, limited in space, limited in time, and by its objectives. War begins with a declaration (of war), ends with a treaty (of peace) or surrender agreement, an act of sharing, etc.
Military necessity, along with distinction, proportionality, humanity (sometimes called unnecessary suffering), and honor (sometimes called chivalry) are the five most commonly cited principles of international humanitarian law governing the legal use of force in an armed conflict.
Military necessity is governed by several constraints: an attack or action must be intended to help in the defeat of the enemy; it must be an attack on a legitimate 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.
Proportionality is a principle under international humanitarian law governing the legal use of force in an armed conflict, whereby belligerents must make sure that the harm caused to civilians or civilian property is not excessive in relation to the concrete and direct military advantage expected by an attack on a legitimate military objective.
Humanity. This principle is based in the Hague Conventions restrictions against using arms, projectiles, or materials calculated to cause suffering or injury manifestly disproportionate to the military advantage realized by the use of the weapon for legitimate military purposes. In some countries, like the United States, weapons are reviewed prior to their use in combat to determine if they comply with the law of war and are not designed to cause unnecessary suffering when used in their intended manner. This principle also prohibits using an otherwise lawful weapon in a manner that causes unnecessary suffering.
Honor is a principle that demands a certain amount of fairness and mutual respect between adversaries. Parties to a conflict must accept that their right to adopt means of injuring each other is not unlimited, they must refrain from taking advantage of the adversary's adherence to the law by falsely claiming the law's protections, and they must recognize that they are members of a common profession that fights not out of personal hostility but on behalf of their respective States.
To fulfill the purposes noted above, the laws of war place substantive limits on the lawful exercise of a belligerent's power. Generally speaking, the laws require that belligerents refrain from employing violence that is not reasonably necessary for military purposes and that belligerents conduct hostilities with regard for the principles of humanity and chivalry.
However, because the laws of war are based on consensus[clarification needed], the content and interpretation of such laws are extensive, contested, and ever-changing.
 The following are particular examples of some of the substance of the laws of war, as those laws are interpreted today.
Some treaties, notably the United Nations Charter (1945) Article 2, and other articles in the Charter, seek to curtail the right of member states to declare war; as does the older Kellogg–Briand Pact of 1928 for those nations who ratified it. Formal declarations of war have been uncommon since 1945 outside the Middle East and East Africa.
Modern laws of war regarding conduct during war (jus in bello), such as the 1949 Geneva Conventions, provide that it is unlawful for belligerents to engage in combat without meeting certain requirements. Article 4(a)(2) of the Geneva Convention relative to the Treatment of Prisoners of War provides that Lawful Combatants are required
Combatants also must be commanded by a responsible officer. That is, a commander can be held liable in a court of law for the improper actions of their subordinates. There is an exception to this if the war came on so suddenly that there was no time to organize a resistance, e.g. as a result of a foreign occupation.
Modern laws of war, specifically within Protocol I additional to the 1949 Geneva Conventions, prohibits attacking people parachuting from an aircraft in distress regardless of what territory they are over. Once they land in territory controlled by the enemy, they must be given an opportunity to surrender before being attacked unless it is apparent that they are engaging in a hostile act or attempting to escape. This prohibition does not apply to the dropping of airborne troops, special forces, commandos, spies, saboteurs, liaison officers, and intelligence agents. Thus, such personnel descending by parachutes are legitimate targets and, therefore, may be attacked, even if their aircraft is in distress.
Modern laws of war, such as the 1949 Geneva Conventions, also include prohibitions on attacking doctors, ambulances or hospital ships displaying a Red Cross, a Red Crescent, Magen David Adom, Red Crystal, or other emblem related to the International Red Cross and Red Crescent Movement. It is also prohibited to fire at a person or vehicle bearing a white flag, since that indicates an intent to surrender or a desire to communicate.
In either case, people protected by the Red Cross/Crescent/Star or white flag are expected to maintain neutrality, and may not engage in warlike acts. In fact, engaging in war activities under a protected symbol is itself a violation of the laws of war known as perfidy. Failure to follow these requirements can result in the loss of protected status and make the individual violating the requirements a lawful target.
The law of war is binding not only upon States as such but also upon individuals and, in particular, the members of their armed forces. Parties are bound by the laws of war to the extent that such compliance does not interfere with achieving legitimate military goals. For example, they are obliged to make every effort to avoid damaging people and property not involved in combat or the war effort, but they are not guilty of a war crime if a bomb mistakenly or incidentally hits a residential area.
By the same token, combatants that intentionally use protected people or property as human shields or camouflage are guilty of violations of the laws of war and are responsible for damage to those that should be protected.
The use of contracted combatants in warfare has been an especially tricky situation for the laws of war. Some scholars claim that private security contractors appear so similar to state forces that it is unclear if acts of war are taking place by private or public agents. International law has yet to come to a consensus on this issue.
After a conflict ends, persons who have committed or ordered any breach of the laws of war, especially atrocities, may be held individually accountable for war crimes through process of law. Also, nations that signed the Geneva Conventions are required to search for, then try and punish, anyone who has committed or ordered certain "grave breaches" of the laws of war. (Third Geneva Convention, Article 129 and Article 130.)
Combatants who break specific provisions of the laws of war are termed unlawful combatants. Unlawful combatants who have been captured may lose the status and protections that would otherwise be afforded to them as prisoners of war, but only after a "competent tribunal" has determined that they are not eligible for POW status (e.g., Third Geneva Convention, Article 5.) At that point, an unlawful combatant may be interrogated, tried, imprisoned, and even executed for their violation of the laws of war pursuant to the domestic law of their captor, but they are still entitled to certain additional protections, including that they be "treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial." (Fourth Geneva Convention Article 5.)