In the Middle Ages the ideals of knighthood restrained some cruelties in warfare, but systematic legal codes did not appear until the 17th cent. The great work of Grotius, De jure belli ac pacis [on the laws of war and peace] (1625) and the works of Vattel had much influence in introducing humane practices. Detailed international treaties governing war are mostly a product of the 19th and 20th cent. The Declaration of Paris (1856; see Paris, Declaration of), the accords concluded at the Hague Conferences (1899, 1907), and the Geneva Conventions (1864, 1906, 1929, 1949) are the main bodies of formulated law.
There is no convention on the laws of war to which all the major powers of the world have acceded, and many conventions provide that their terms shall be inoperative if any of the belligerents is not a signatory or if an enemy commits a violation. Despite such provisions, many nations have adopted the laws of war, and the conditions of warfare have undoubtedly been ameliorated, particularly in the treatment of prisoners and the consideration shown to the sick and wounded. The care of the sick and the wounded is facilitated by making medical personnel noncombatants and by clearly marking hospitals and similar installations, thus sparing them from attack. Conventions restricting the use of certain weapons probably have not materially mitigated the horrors of war. For the most part, only those weapons that are of limited military use, e.g., poison gas, have been effectively banned, while efforts to prohibit militarily effective weapons, e.g., atomic weapons and submarine mines, have not succeeded.
The laws of war have had as their objective the protection of civilian populations by limiting all action to the military. A distinction was made between combatants and noncombatants, the former being defined in terms of traditional military units. Thus combatants must have a commander responsible for subordinates, wear a fixed and recognizable emblem, carry arms openly, and follow the laws of war. But the development of aerial bombing in World War I and of guerrilla forces dependent on civilians has tended to make all enemy territory part of the theater of operations. New practices and categories have yet to be worked out to protect civilian centers adequately.
Civilians in territory occupied by the enemy are, however, supposed to be entitled to certain protections. There may not be imprisonment without cause, and fines may not be levied upon a whole civilian population for individual offenses. Private property also receives limited protection, and it may not be confiscated for military use unless fair compensation is paid. Special rules govern such actions against property as the taking of a prize at sea or in port, the confiscation of contraband, and the use of the blockade. Property destroyed in the course of action against the enemy is, of course, not compensable. Places of religious, artistic, or historical importance should not be attacked unless there is military need.
No direct diplomatic relations exist between belligerents, but neutral diplomats are often given custody of property in enemy territory and are entrusted with negotiations. In the field of combat, passports, safe-conducts, and flags of truce permit consultations between opposing commanders. Hostilities may even be totally suspended by an armistice, which is often the prelude to surrender.
Violations of the laws of war have probably occurred in all major conflicts; a nation confident of victory will frequently not be deterred even by fear of reprisals. After World War II the military and civilian leaders of the Axis Powers who were responsible for violations were tried for war crimes, and some Americans were tried for war crimes in the Vietnam War (see My Lai incident).
See M. Greenspan, The Modern Law of Land Warfare (1959) and The Soldier's Guide to the Laws of War (1969); S. D. Bailey, Prohibitions and Restraints in War (1972).
Positive international humanitarian law consists of treaties (international agreements) which directly affect the laws of war by binding consenting nations and achieving widespread consent, including:
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.
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, whereby 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 impact of the use of such projectiles and it is possible that, in future, there will be a consensus view in international legal circles that use of such projectiles violate general principles of the law applicable to use of weapons in armed conflict. This is because in future it may be the consensus view that depleted uranium projectiles breaches 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.
Some of the central principles underlying laws of war are:
To this end, laws of war are intended to mitigate the evils of war by:
It is a violation of the laws of war to engage in combat without meeting certain requirements, among them the wearing of a distinctive uniform or other distinctive signs visible at a distance, and the carrying of weapons openly. Impersonating soldiers of the other side by wearing the enemy's uniform is allowed, though fighting in that uniform, like fighting under a white flag, is perfidy which is forbidden, as is the taking of hostages.
The law of war is derived from two principal sources:
By the same token, combatants that use protected people or property as shields or camouflage are guilty of violations of laws of war and are responsible for damage to those that should be protected.
Soldiers who break specific provisions of the laws of war lose the protections and status afforded as prisoners of war but only after facing a "competent tribunal" (GC III Art 5). At that point they become an unlawful combatant but they must still be "treated with humanity and, in case of trial, shall not be deprived of the rights of fair and regular trial", because they are still covered by GC IV Art 5. For example in 1976 foreign soldiers fighting for FNLA were captured by the MPLA in the civil war that broke out when Angola gained independence from Portugal in 1975. After "a regularly constituted court" found them guilty of being mercenaries, three Britons and an American were shot by a firing squad on July 10, 1976. Nine others were imprisoned for terms of 16 to 30 years.
Spies and terrorists may be subject to civilian law or military tribunal for their acts and in practice have been subjected to torture and/or execution. The laws of war neither approve nor condemn such acts, which fall outside their scope. However, nations that have signed the UN Convention Against Torture have committed themselves not to use torture on anyone for any reason. Citizens and soldiers of nations which have not signed the Fourth Geneva Convention are also not protected by it (Article 4: "Nationals of a State which is not bound by the Convention are not protected by it".), whether they are spies or terrorists. Also, citizens and soldiers of nations which have not signed and do not abide by the Third and Fourth Geneva Conventions are not protected by them. (Article 2, of both Conventions: "[The High Contracting Parties] shall furthermore be bound by the Convention in relation to [a Power which is not a contracting party], if the latter accepts and applies the provisions thereof". note: emphasis added)
If someone is (or is suspected to be) a citizen or soldier of a nation which has signed or abides by the Fourth Geneva Convention (see Art. 2 and Art. 4 citations above), or is (or is suspected to be) a "prisoner of war" (POW) per the definitions of such "protected persons" in the Third Geneva Convention (see Art. 4 and Art. 5), the following applies: A POW who breaks specific provisions of the laws of war may be penalized, but not penalized worse than the tribunal would penalize its own soldiers for the same offense (and usually a disciplinary, not judicial, punishment if its own soldiers normally wouldn't be brought to trial for a particular offense) and POW's may not be penalized based on rank or gender, nor with corporal punishment, collective punishments for individual acts, lack of daylight, or torture/cruelty (GC IV, Art. 82 through Art. 88).
After a conflict has ended, 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 which 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. (see GC III, Art. 129 and Art. 130)
History has shown that the laws of war are traditionally more strictly applied to those defeated, as the victorious faction are placed in the role of policing themselves. While it can be argued that the victors may be less strict on their own forces, it can also be argued that the signing of the treaties involved in the laws of war implies a good-faith promise to adhere to them equally. As with many facets of war, the aftermath and subsequent legal proceedings depend heavily on circumstance, and are different for each conflict.
There is an emerging trend in the US to hold private corporations civilly liable for aiding and abetting in war crimes, by knowingly providing substantial assistance in the commission of the crimes. Under international law, the mens rea element is knowledge, not intent that the crimes be carried out. This opens the door not only to hold private security contractors liable, but also other kinds of corporations which employ violent mercenary or terrorist groups as private security forces. Although conflict zones often lack functioning legal systems, and government may even have passed laws immunizing private mercenaries from criminal liability, aiding and abetting a war crime can still be the basis for civil liability in a foreign court with jurisdiction over the defendant corporation.