The law of war refers to the component of international law that regulates the conditions for 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 international law.
Among other issues, modern laws of war address 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.
Early sources and history
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:
An example from the Book of Deuteronomy 20:19–20 limits the amount of acceptable collateral and environmental damage:
Also, Deuteronomy 20:10–12, requires the Israelites to make an offer of peace to the opposing party before laying siege to their city.
Similarly, Deuteronomy 21:10–14 requires that female captives who were forced to marry the victors of a war could not be sold as slaves.
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.
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:
- Lawmaking treaties (or conventions) — see § International treaties on the laws of war below.
- Custom. Not all the law of war derives from or has been incorporated in such treaties, which can refer to the continuing importance of customary law as articulated by the Martens Clause. Such customary international law is established by the general practice of nations together with their acceptance that such practice is required by law.
- General Principles. "Certain fundamental principles provide basic guidance. For instance, the principles of distinction, proportionality, and necessity, all of which are part of customary international law, always apply to the use of armed force".
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 in the Venezuelan city of Trujillo in November 25 and 26 1820 between the president of the Republic of Colombia, Simon Bolivar 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.
Purposes of the laws
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 felt that codifying laws of war would be beneficial.
Some of the central principles underlying laws of war are:
- Wars should be limited to achieving the political goals that started the war (e.g., territorial control) and should not include unnecessary destruction.
- Wars should be brought to an end as quickly as possible.
- People and property that do not contribute to the war effort should be protected against unnecessary destruction and hardship.
To this end, laws of war are intended to mitigate the hardships of war by:
- Protecting both combatants and non-combatants from unnecessary suffering.
- Safeguarding certain fundamental human rights of persons who fall into the hands of the enemy, particularly prisoners of war, the wounded and sick, children, and civilians.
- Facilitating the restoration of peace.
Principles of the laws of war
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.
Example substantive laws of war
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, 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.
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, such as wearing distinctive uniform or other distinctive signs visible at a distance, carrying weapons openly, and conducting operations in accordance with the laws and customs of war. Impersonating enemy combatants by wearing the enemy's uniform is allowed, though fighting in that uniform is unlawful perfidy, as is the taking of hostages.
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 his or her 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, The 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.
Applicability to states and individuals
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.
Remedies for violations
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.)
For example, in 1976, foreign soldiers fighting for the National Liberation Front of Angola (FNLA) were captured by the People's Movement for the Liberation of Angola (MPLA) during the Angolan Civil War in 1975. In the Luanda Trial, after "a regularly constituted court" found them guilty of being unlawful 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.
International treaties on the laws of war
- 1856 Paris Declaration Respecting Maritime Law abolished privateering.
- 1864 Geneva Convention for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field.
- 1868 St. Petersburg Declaration Renouncing the Use of Explosive projectiles Under 400 grams Weight.
- 1874 Project of an International Declaration concerning the Laws and Customs of War (Brussels Declaration). Signed in Brussels 27 August. This agreement never entered into force, but formed part of the basis for the codification of the laws of war at the 1899 Hague Peace Conference.
- 1880 Manual of the Laws and Customs of War at Oxford. At its session in Geneva in 1874 the Institute of International Law appointed a committee to study the Brussels Declaration of the same year and to submit to the Institute its opinion and supplementary proposals on the subject. The work of the Institute led to the adoption of the Manual in 1880 and it went on to form part of the basis for the codification of the laws of war at the 1899 Hague Peace Conference.
- 1899 Hague Conventions consisted of three main sections and three additional declarations: I – Pacific Settlement of International Disputes II – Laws and Customs of War on Land III – Adaptation to Maritime Warfare of Principles of Geneva Convention of 1864 Declaration I – On the Launching of Projectiles and Explosives from Balloons Declaration II – On the Use of Projectiles the Object of Which is the Diffusion of Asphyxiating or Deleterious Gases Declaration III – On the Use of Bullets Which Expand or Flatten Easily in the Human Body
- 1907 Hague Conventions had thirteen sections, of which twelve were ratified and entered into force, and two declarations: I – The Pacific Settlement of International Disputes II – The Limitation of Employment of Force for Recovery of Contract Debts III – The Opening of Hostilities IV – The Laws and Customs of War on Land V – The Rights and Duties of Neutral Powers and Persons in Case of War on Land VI – The Status of Enemy Merchant Ships at the Outbreak of Hostilities VII – The Conversion of Merchant Ships into War-ships VIII – The Laying of Automatic Submarine Contact Mines IX – Bombardment by Naval Forces in Time of War X – Adaptation to Maritime War of the Principles of the Geneva Convention XI – Certain Restrictions with Regard to the Exercise of the Right of Capture in Naval War XII – The Creation of an International Prize Court [Not Ratified] XIII – The Rights and Duties of Neutral Powers in Naval War Declaration I – extending Declaration II from the 1899 Conference to other types of aircraft Declaration II – on the obligatory arbitration
- 1909 London Declaration concerning the Laws of Naval War largely reiterated existing law, although it showed greater regard to the rights of neutral entities. Never went into effect.
- 1922 The Washington Naval Treaty, also known as the Five-Power Treaty (6 February).
- 1923 Hague Draft Rules of Aerial Warfare. Never adopted in a legally binding form.
- 1925 Geneva protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare.
- 1927–1930 Greco-German arbitration tribunal.
- 1928 Kellogg-Briand Pact (also known as the Pact of Paris).
- 1929 Geneva Convention, Relative to the treatment of prisoners of war.
- 1929 Geneva Convention on the amelioration of the condition of the wounded and sick.
- 1930 Treaty for the Limitation and Reduction of Naval Armament (22 April).
- 1935 Roerich Pact.
- 1936 Second London Naval Treaty (25 March).
- 1938 Amsterdam Draft Convention for the Protection of Civilian Populations Against New Engines of War. This convention was never ratified.
- 1938 League of Nations declaration for the "Protection of Civilian Populations Against Bombing From the Air in Case of War."
- 1945 United Nations Charter (entered into force on October 24, 1945).
- 1946 Judgment of the International Military Tribunal at Nuremberg.
- 1947 Nuremberg Principles formulated under UN General Assembly Resolution 177 21 November 1947.
- 1948 United Nations Convention on the Prevention and Punishment of the Crime of Genocide.
- 1949 Geneva Convention I for the Amelioration of the Condition of the Wounded and Sick in Armed Forces in the Field.
- 1949 Geneva Convention II for the Amelioration of the Condition of Wounded, Sick and Shipwrecked Members of Armed Forces at Sea.
- 1949 Geneva Convention III Relative to the Treatment of Prisoners of War.
- 1949 Geneva Convention IV Relative to the Protection of Civilian Persons in Time of War.
- 1954 Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict.
- 1971 Zagreb Resolution of the Institute of International Law on Conditions of Application of Humanitarian Rules of Armed Conflict to Hostilities in which the United Nations Forces May be Engaged.
- 1974 United Nations Declaration on the Protection of Women and Children in Emergency and Armed Conflict.
- 1977 United Nations Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques.
- 1977 Geneva Protocol I Additional to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of International Armed Conflicts.
- 1977 Geneva Protocol II Additional to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of Non-International Armed Conflicts.
- 1978 Red Cross Fundamental Rules of International Humanitarian Law Applicable in Armed Conflicts.
- 1980 United Nations Convention on Prohibitions or Restrictions on the Use of Certain Conventional Weapons Which May be Deemed to be Excessively Injurious or to Have Indiscriminate Effects (CCW). 1980 Protocol I on Non-Detectable Fragments. 1980 Protocol II on Prohibitions or Restrictions on the Use of Mines, Booby-Traps and Other Devices. 1980 Protocol III on Prohibitions or Restrictions on the Use of Incendiary Weapons. 1995 Protocol IV on Blinding Laser Weapons. 1996 Amended Protocol II on Prohibitions or Restrictions on the Use of Mines, Booby-Traps and Other Devices. Protocol on Explosive Remnants of War (Protocol V to the 1980 Convention), 28 November 2003, entered into force on 12 November 2006.
- 1994 San Remo Manual on International Law Applicable to Armed Conflicts at Sea.
- 1994 ICRC/UNGA Guidelines for Military Manuals and Instructions on the Protection of the Environment in Time of Armed Conflict.
- 1994 UN Convention on the Safety of United Nations and Associated Personnel.
- 1996 The International Court of Justice advisory opinion on the Legality of the Threat or Use of Nuclear Weapons.
- 1997 Ottawa Treaty - Convention on the Prohibition of the Use, Stockpiling, Production and Transfer of Anti-Personnel Mines and on their Destruction.
- 1998 Rome Statute of the International Criminal Court (entered into force 1 July 2002).
- 2000 Optional Protocol on the Involvement of Children in Armed Conflict (entered into force 12 February 2002).
- 2005 Geneva Protocol III Additional to the Geneva Conventions of 12 August 1949, and Relating to the Adoption of an Additional Distinctive Emblem.
- 2008 Convention on Cluster Munitions (entered into force 1 August 2010).
- Self-defence in international law
- Customary international humanitarian law
- List of Articles of War
- List of weapons of mass destruction treaties
- Arms control (includes list of treaties)
- Rule of Law in Armed Conflicts Project (RULAC)
- Command responsibility
- Crimes against humanity
- International law
- Islamic military jurisprudence
- Law of occupation
- Law of the Sea
- Right of conquest
- Total war
- Journal of International Law of Peace and Armed Conflict
- Targeted killing
- War crime, an act that amounts to a violation of the law of war