International humanitarian law, also called the law of armed conflict or the law(s) of war, is the branch of international law that regulates conduct in an armed conflict.
International humanitarian law strives to limit the effects of armed conflicts. On the one hand, it provides for the protection of those who are not or no longer participating in hostilities, such as civilians, the wounded, sick and shipwrecked, or captured combatants. On the other hand, international humanitarian law limits the means and methods of warfare, or, in other words, the weapons and tactics that may be used. In particular, it prohibits means and methods of warfare that do not discriminate between those who are not or no longer participating in hostilities and those who do. Those who are participating in hostilities are protected by the prohibition of using means or methods of warfare that are of a nature to cause superfluous injury or unnecessary suffering.
International humanitarian law binds all parties to an armed conflict, including non-state armed groups. In this respect, international humanitarian law is different from international human rights law which primarily binds states. However, there are different explanations why and how international humanitarian law binds non-state armed groups, see for example J. Kleffner, ‘The Applicability of International Humanitarian Law to Organized Armed Groups’, 93 (882) International Review of the Red Cross, 2011, p 443. Pursuant to the principle of equality of belligerents, international humanitarian law applies equally to all parties to an armed conflict: Every party to an armed conflict has the same rights and obligations under international humanitarian law, not more or less obligations depending on whether or not their resort to force is lawful or not. In other words, international humanitarian law applies independently from any considerations concerning the lawfulness to use force. This separation between the application of international humanitarian law and the legal regime governing the use of force is necessary for both humanitarian and practical reasons. On the one hand, at least the parties to a conflict will always disagree on who resorted to force unlawfully or whose cause is just. On the other hand, those affected by the conflict are in need of the same protection.
The core of international humanitarian law rules can be found in the four 1949 Geneva Conventions and their two 1977 Additional Protocols. The four 1949 Geneva Conventions seek to protect those who do not, or no longer actively participate in hostilities, namely wounded and sick members of armed forces in land warfare, sick, wounded and shipwrecked members of armed forces at sea, prisoners of war, and civilians. Protected persons shall in all circumstances be treated humanely and cared for. The Geneva Conventions are universally ratified. The 1977 Additional Protocol I to the 1949 Geneva Conventions relating to the protection of victims in international armed conflicts and the 1977 Additional Protocol II to the 1949 Geneva Conventions relating to the protection of victims in non-international armed conflicts further develop the protection of those not or no longer participating in hostilities. In addition, the two 1977 Additional Protocols provide for rules on means and methods of warfare. Before the adoption of the 1977 Additional Protocols, rules governing the conduct of hostilities could only be found in the so-called ‘Hague Law’, in particular the fourth Hague Convention respecting the Laws and Customs of War on Land, with its annexed Regulations concerning the Laws and Customs of War on Land of 1907. With the adoption of the two 1977 Additional Protocols, two different strands of sources for international humanitarian law merged: the so-called ‘Geneva Law’ , i.e. the rules for the protection of victims of war, and the so-called ‘Hague Law’ with rules affecting the conduct of hostilities. Yet, the 1907 Hague Regulations remain applicable in a wide array of situations and its provisions on prohibited means and methods of warfare and for occupied territories continue to be relevant. Finally, there are several treaties dedicated to specific topics. A series of such treaties prohibit or restrict the use of certain weapons, such as biological weapons, chemical weapons, incendiary weapons, blinding laser weapons, anti-personnel mines, and cluster munitions. Other treaties provide for the specific protection of cultural property or the environment.
Customary international law complements the protection provided by treaty law. Customary international law consists of unwritten rules that come from a general practice accepted as law. It exists independently from treaty law. Customary international humanitarian law applies universally and binds all parties to a conflict, independently from the application of treaty law. In particular, customary international humanitarian law governing non-international armed conflicts is more detailed than treaty law governing non-international armed conflicts. The ICRC maintains a searchable database that identifies the rules of customary international humanitarian law.
International humanitarian law applies only to armed conflicts. It does not cover other situations of violence, such as riots or other internal tensions or disturbances. International humanitarian law distinguishes between international armed conflicts and armed conflicts of non-international character. A wide array of provisions, including those in the four 1949 Geneva Conventions and the 1977 Additional Protocol I, govern international armed conflicts, i.e. conflicts between at least two States. Non-international armed conflicts are subject to a more limited set of rules provided for in common Article 3 to the four 1949 Geneva Conventions. Article 3 common to the four Geneva Conventions of 1949 provides minimal guarantees of protection for those who do not or no longer participate in hostilities. and in the 1977 Additional Protocol II. The two regimes are converging under customary international law, in particular in relation to the conduct of hostilities. Yet, important differences remain, in particular for the status of persons. During international armed conflicts, combatants have so-called combatant immunity: they are immune from prosecutions for belligerent acts that comply with international humanitarian law, and they are entitled to prisoner of war status when captured. Generally, combatants are defined as members of the armed forces of a party to an international armed conflict. However, other persons, provided that they meet the relevant criteria enshrined in Article 4 of the Third Geneva Convention of 1949 and Article 44 of Additional Protocol I may also fall within this category. In contrast, fighters during a non-international armed conflict, although still entitled to legal protection, may be prosecuted for mere participation in hostilities under domestic law, including for acts that comply with international humanitarian law.
International humanitarian law strives to limit the effects of armed conflict and minimize human suffering. International humanitarian law rests on a careful balancing between the foundational principles of humanity and military necessity. The principle of military necessity can be both limiting and permissive. As a limiting principle, military necessity restrains the lawful use of force to what is necessary to achieve a military goal and prohibits violence excessive to achieve this aim. This conception is expressed in the 1868 Saint Petersburg Declaration: ‘That the only legitimate object which States should endeavour to accomplish during war is to weaken the military forces of the enemy; That for this purpose it is sufficient to disable the greatest possible number of men; That this object would be exceeded by the employment of arms which uselessly aggravate the sufferings of disabled men, or render their death inevitable … .’ The principle of military necessity operates as permissive principle that allows death and destruction when necessary from a military point of view. Yet, the principle of military necessity cannot be invoked to justify violations of international humanitarian law.
Military necessity is counter-balanced by the principle of humanity. The principle of humanity protects those who are not or no longer actively participating in hostilities and provides for their humane treatment at all times. Furthermore, the principle of humanity protects those who directly participate in hostilities from superfluous injury or unnecessary suffering. The Martens clause is an expression of the principle of humanity. Its original formulation in the Preamble of the 1899 II Convention with Respect to the Laws and Customs of War by Land and its Annex, Regulations Respecting the Laws and Customs of War on Land provides that: ‘Until a more complete code of the laws of war is issued, the High contracting parties think it right to declare that in cases not included in the Regulations adopted by them, populations and belligerents remain under protection and empire of the principles of international law, as they result from the usages established between civilized nations, from the laws of humanity, and the requirements of public conscience’.
From these two foundational principles flow a series of other basic principles upon which international humanitarian law rests. The principle of distinction requires parties to an armed conflict to distinguish between combatants and civilians and between military and civilian objects. Attacks against lawful military objectives are prohibited if the attack ‘may be expected’ to result in excessive civilian harm under the principle of proportionality.