IHL regulates the conduct of hostilities by a number of means, including rules regarding permissible targets, restrictions on permissible weapons, and rules on allowable methods of warfare. Three general principles can be observed.
- The choice of means and methods of warfare is not unlimited.
- The use of means and methods of a nature to cause unnecessary suffering or superfluous injury are prohibited.
- The only legitimate object of war is to overpower or weaken the enemy forces in order to compel the surrender of the enemy, not to destroy and kill as many members of the opposing forces as possible.
Basic Rules on Targeting
The most basic rule of IHL is that civilians and civilian objects are not to be made the object of direct attack, though some proportionate collateral damage is considered unavoidable and therefore not illegal. Certain rules have been developed to assist parties to the conflict with regards to targeting decisions. Combatants are legitimate targets of attack, for as long as they take direct part in hostilities. Once they are rendered hors de combat by injury, sickness, or by surrendering, they are immune from attack, but may be taken as prisoners of war. With regard to objects which may be attacked, the rule is defined in Article 52(2) of Protocol I, which states that military objectives are lawful objects of attack. Military objectives are defined as:
Those objects which by their nature, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralisation, in the circumstances ruling at the time, offers a definite military advantage.
What amounts to a ‘definite military advantage’ is a decision to be made by the commander of the forces involved, according to the conditions in place at the time of attack. The commander must also decide whether an attack on a legitimate military target might result in incidental damage to civilians and civilian objects that would be excessive or disproportionate to the military objective sought. Such an attack would then be considered illegal, as it would be ‘indiscriminate’ under Article 51(5)(b) of Protocol I; so the commander should not authorise such an attack. Certain objects will always be considered immune from attack – such as schools, houses and places of worship. However, these places will lose their immunity from attack if used for military purposes, or to launch military attacks.
Prohibitions on Weapons
As a general rule, certain weapons will be considered illegal under IHL if they are ‘incapable of distinguishing between civilian and military targets’ (Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons  ICJ Rep 226 at 257). An additional restriction on weaponry is outlined in the principle regarding unnecessary suffering. Protocol I, Article 35(2) provides that ‘it is prohibited to employ weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering’. There are numerous weapons that have been banned or strictly limited in use on this basis.
- Poison– article 23(a) of the Hague Regulations forbids using poison or poisoned weapons.
- Certain projectiles – the 1868 St Petersburg Declaration bans the use of projectiles that weigh less than 400 g and that are either explosive or charged with ‘fulminating or inflammable substances’. Hague Declaration IV of 1899 bans the use of expanding bullets, which are designed to flatten easily upon contact with the human body, resulting in significantly larger wounds.
- Non-detectable fragments – Protocol I of the Conventional Weapons Convention (CWC) prohibits the use of weapons that injure primarily through use of fragments that cannot be detected by means of X-ray, such as plastic or glass.
- Booby-traps– Protocol II of the CWC prohibits the use of booby-traps attached to ‘harmless portable objects’ specifically designed to detonate when approached or disturbed. The Protocol also bans the placing of booby-traps attached to or associated with the protective emblems such as the Red Cross; sick, wounded or dead persons; burial or cremation sites; medical facilities, equipment, supplies or transportation; children’s toys and other portable objects specially connected to children such as objects for feeding, health, hygiene, clothing or education; food or drink, kitchen utensils and appliances (excluding those in military installations); animals and/or animal carcasses; and objects of religious, historical or cultural significance.
- Landmines– certain types of landmines are banned under both Protocol II to the CWC and under the Ottawa Convention. Remotely-delivered mines (mines whose location cannot be recorded in a designated minefield) are banned, as well as mines equipped to detonate in response to the deployment of mine detectors. Mines which are designed to detonate even after deactivation are also banned, as well as certain types of anti-personnel mines. The Ottawa Convention goes further than Protocol II to the CWC, in that it bans the use, stockpiling, production and transfer of all anti-personnel mines.
- Incendiary weapons– Protocol III to the CWC prohibits the use of incendiary devices – such as flame-throwers or napalm – against civilians or civilian objects. The Protocol also bans use of such devices against military installations if they are located in a concentration of civilians or civilian objects.
- Blinding laser weapons – Protocol IV to the CWC prohibits the use of weapons whose sole combat function is to cause permanent blindness. Weapons that cause temporary blindness – such as ‘flash-bang’ grenades – are permitted.
- Cluster munitions and explosive remnants of war – Protocol V to the CWC provides that States parties must endeavour to clear their territory of explosive remnants of war, such as cluster munitions, landmines, and other unexploded ordnance. The protocol does not ban the use of such weapons. The use of certain types of cluster munitions is prohibited under the 2008 Convention on Cluster Munitions.
- Chemical weapons– The Hague Regulations prohibit the use of projectiles the sole object of which is the diffusion of asphyxiating or deleterious gases. Gas warfare is also prohibited in the 1925 Geneva Gas Protocol and the 1993 Convention on Chemical Weapons. The 1993 Convention also bans the development, stockpiling and production of chemical weapons.
- Biological weapons– The 1925 Geneva Gas Protocol bans the use of bacteriological agents in warfare, as does the 1972 Convention, which prohibits the development, production and stockpiling of bacteriological (biological) and toxin weapons.
- Nuclear weapons– nuclear weapons are not subject to any international humanitarian law treaty regarding their use. The question of whether such weapons could ever be legal under international law was addressed by the International Court of Justice in an advisory opinion in 1996. The Court found that IHL would indeed apply to nuclear weapons but that ‘there is in neither customary nor conventional international law any comprehensive and universal prohibition of the threat or use of nuclear weapons as such. (Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons  ICJ Rep 226 at 266)
Rules on Methods of Warfare
IHL provides that parties to the conflict take precautions when launching attacks. Under Article 57(2) of Protocol I, those who plan attacks must take certain precautions such as:
- verifying that the objectives to be attacked are military;
- choosing the means and methods of attack that are most likely to avoid or minimise incidental injury to civilians or civilian objects; and
- refraining from launching attacks likely to violate the principle of proportionality.
In addition to these general principles, there are certain specific rules regarding permitted and prohibited methods of warfare.
- Orders of ‘no quarter’- it is prohibited in all types of armed conflict for a commander to order that ‘no quarter’ will be given; i.e. that there will be no survivors and that no prisoners will be taken. Such an order is a war crime.
- Siege warfare and starvation of civilians– Article 54 of Protocol I provides that starvation of civilians is prohibited as a method of warfare. It is prohibited to attack, destroy, remove or render useless objects indispensable to the survival of the civilian population, such as foodstuffs, agricultural areas, livestock, drinking water installations and supplies, and irrigation works. Siege warfare is not prohibited in and of itself, but steps must be taken to differentiate between sieges of military installations, and sieges of civilian areas.
- Protection of cultural property– cultural property and places of worship are to be specially protected. The 1954 Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict provides that movable and immovable property of great importance to the cultural heritage of every people – such as monuments of architecture, art or history, whether religious or secular; archaeological sites; buildings or historic or artistic importance; works of art; manuscripts and books of artistic, historical or archaeological interest; as well as museums, libraries and other depositories – are not to be made the objects of attack. Parties to the Convention are required to ensure that such cultural objects are not used in a manner which would expose such objects to destruction or damage in the event of armed conflict. Additional protections for cultural and religious property are found in Article 53 of Protocol I; a Second Protocol to the 1954 Hague Convention extended these international protections to non-international armed conflicts.
- Works and installations containing dangerous forces– certain installations, such as dams, dykes, or nuclear power plants are specially protected from attack, because the partial or total destruction of such installations would likely result in catastrophic damage. Thus, under Article 56 of Protocol I, works or installations containing dangerous forces are not to be made the object of attack, even if those installations are military objectives, if such an attack might cause the release of dangerous forces and result in severe losses among the civilian population. By the same token, military installations near such dangerous forces should also be immune from attack, for similar reasons. As such, parties to the Protocol should refrain from location such works and installations near military objectives.
- Environmental protections– Article 55 of Protocol I provides some guidance on protection of the natural environment during armed conflict, stating that ‘care should be taken in warfare to protect the natural environment against widespread, long-term and severe damage’. These criteria are cumulative. The ENMOD Convention takes a somewhat different approach, and prohibits modification of the environment as a method of warfare.
- Perfidy– Additional Protocol I, codifying customary international law, prohibits acts of perfidy. Perfidy is an act that invites the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of IHL, after which point, the person committing the act of perfidy either kills, injures or captures the person deceived. Perfidious acts include feigning surrender or negotiation under a flag of truce; feigning incapacitation due to wounds or illness; feigning civilian, non-combatant status; misusing the emblem, or feigning protected status through use of the signs, emblems or uniforms of the UN, of neutral States, or of any other States not party to the conflict. The essence of perfidy is the intentional misuse of a protected status to invite or encourage immunity from attack, after which point, an attack is launched. Ruses of war – for instance, faking retreat, camouflaging troops, using mock or dummy installations and machinery – are not considered to be perfidy and thus are not prohibited.
- Espionage– espionage is not prohibited under IHL, but spies are not granted combatant immunity like members of the armed forces. A person captured while engaging in an act of espionage may be tried under the domestic legislation of the State in which they are captured.
- Mercenaries– the use of mercenaries is prohibited under Article 47 of Additional Protocol I. Article 47 outlines six cumulative criteria for a person to be designated as a mercenary, namely that such person:
- is specially recruited locally or abroad in order to fight in an armed conflict;
- does, in fact, take a direct part in the hostilities;
- is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
- is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;
- is not a member of the armed forces of a Party to the conflict; and
- has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.
- Pillage– the act of pillage – the looting or plundering of individuals and/or locations – is prohibited in a number of contexts. Pillage of military wounded and sick is prohibited under Article 15 of Convention I and Article 18 of Convention II. Pillage of civilian wounded and sick is prohibited under Article 16 of Convention IV. Pillage against towns and other locations is prohibited under the Hague Regulations (Articles 28 and 47) and is considered a war crime under Article 8(2)(b)(xvi) of the Rome Statute of the International Criminal Court.
- Belligerent reprisals– belligerent reprisals are actions which would normally be contrary to the laws of war but which are justified because they are taken by one party to an armed conflict against another party to an armed conflict, in response to the latter’s violation of IHL. Customary international law regulates belligerent reprisals requiring the fulfillment of five conditions:
- protests or other attempts to secure compliance with IHL must first be undertaken;
- a warning must be issued before resort to belligerent reprisal;
- the reprisal must be proportionate to the original breach of IHL;
- decisions to launch belligerent reprisals must only come from higher authorities, and not from individual combatants; and
- once the enemy ceases its violation of IHL, the belligerent reprisal must also cease.
There are certain persons and objects against which belligerent reprisals may never be launched – these include the wounded, sick and shipwrecked, prisoners of war, civilians and the civilian population, civilian objects, cultural property, the natural environment, works and installations containing dangerous forces, and objects indispensable to the survival of the civilian population. The extensive number of prohibitions on permissible belligerent reprisals limited the practice of belligerent reprisals.