There are three basic rules that regulate the way in which a party to an armed conflict may carry out military operations, i.e. conduct hostilities. These are the rules on distinction, proportionality and precautions. They aim to protect civilians against the effect of hostilities. In addition to these rules, there is the prohibition against causing superfluous injury or unnecessary suffering, which protects combatants and other legitimate targets of attack. These rules have been codified notably in Additional Protocol I. They exist in customary IHL for international and non-international armed conflicts.
The basic rule of distinction requires that the parties to an armed conflict distinguish at all times between civilian persons and civilian objects on the one hand, and combatants and military objectives on the other. A party to an armed conflict may direct an attack only against combatants or military objectives. Neither the civilian population nor individual civilians may be attacked unless and for such time as they directly participate in hostilities (see box). Attacks must be strictly limited to military objectives and may not be directed against civilian objects. In so far as objects are concerned, military objectives are limited to those objects that by their nature, location, purpose or use make an effective contribution to military action and whose partial or total destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage. Typical military objectives are establishments, buildings and positions where enemy combatants, and their matériel and armaments, are located, and military means of transportation and communication. When civilian objects are used for military purposes (e.g. a civilian train that is used to transport weapons and combatants) they may be regarded as military objectives.
The prohibition against indiscriminate attacks is derived from the principle of distinction. Indiscriminate attacks are:
• those that are not directed at a specific military objective (e.g. a soldier firing in all directions without aiming at a particular military objective, thus endangering civilians)
• those that employ a method or means of warfare that cannot be directed at a specific military objective (e.g. long-range missiles that cannot be aimed precisely at their targets)
• those that employ a method or means of warfare, the effects of which cannot be limited (e.g. a 10-tonne bomb used to destroy a single building).
Attacks directed against a combatant or a military objective must be in accordance with the proportionality rule. This means that it is prohibited to launch an attack that is likely to cause incidental loss of civilian life, injury to civilians, and/or damage to civilian objects that would be excessive in relation to the concrete and direct military advantage anticipated. In other words, a military objective may be attacked only after an assessment leading to the conclusion that civilian losses are not expected to outweigh the military advantage foreseen.
A party to an armed conflict must take constant care to spare civilians or civilian objects when carrying out military operations. The party conducting an attack must do everything feasible to verify that the targets are military objectives. It must choose means and methods of attack that avoid, or at least keep to a minimum, the incidental harm to civilians and civilian property. It must refrain from launching an attack if it seems clear that the losses or damage caused would be excessive in relation to the concrete and direct military advantage anticipated. Effective warning must be given of attacks that may affect the civilian population, unless circumstances do not permit. Precautions must also be taken against the effects of attacks. For example, military objectives must not, as far as possible, be situated in the vicinity of civilian populations and civilian objects; all other necessary precautions must also be taken.
Prohibition against causing superfluous injury or unnecessary suffering
Employing weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering is prohibited. This prohibition refers specifically to combatants: it says that weapons of certain kinds are prohibited because they harm combatants in unacceptable ways. Although the rule is generally accepted, there is disagreement about the proper way to decide whether a weapon causes superfluous injury or unnecessary suffering. The International Court of Justice defined unnecessary suffering as “harm greater than that unavoidable to achieve legitimate military objectives” (Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996). For instance, the rule against targeting soldiers’ eyes with lasers, as laid down in Protocol IV to the Convention on Certain Conventional Weapons was inspired by the belief that deliberately causing permanent blindness in this fashion amounted to the infliction of superfluous injury or unnecessary suffering.
Direct participation in hostilities
Civilians are protected against attacks, unless and for such time as they directly participate in hostilities. To clarify what this means in practice, the ICRC conducted several meetings of experts at which this notion was discussed. In 2009, the ICRC published a document based on these discussions: Interpretive Guidance on the Notion of Direct Participation in Hostilities under International Humanitarian Law. Interpretive Guidance stipulates that civilians are considered to be participating directly in hostilities when they carry out specific acts as part of the conduct of hostilities between parties to an armed conflict. In order to qualify as direct participation in hostilities, a specific act must meet the following criteria cumulatively:
1. The act must reach a certain threshold of harm. This is the case when the act will likely adversely affect the military operations or military capacity of a belligerent party. It could also be the case when the act will likely injure or kill civilians or render combatants hors de combat or will destroy civilian objects.
2. There must be a direct causal link between the act and the harm likely to result either from that act or from a coordinated military operation of which that act constitutes an integral part.
3. There must be a belligerent nexus. This means that the act must be specifically designed to directly cause the required threshold of harm in support of a belligerent party and to the detriment of another.
Civilian are regarded as directly participating in hostilities, and lose their protection against attack, if and for as long as they carry out such acts.
Moreover, measures preparatory to the execution of a specific act that constitutes direct participation in hostilities, as well as the deployment to and the return from the location of its execution, are included in the concept of direct participation in hostilities.