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War Watch
Legal Explainers

Means and Methods of Warfare

A means of warfare is a weapon or weapon platform. A method of warfare is the manner in which a weapon is used.20 In addition to the general rules on the conduct of hostilities, the use of weapons is subject to two general principles of a customary law nature.1S. Casey-Maslen, ‘Weapons’, in B. Saul and D. Akande (eds), The Oxford Guide to International Humanitarian Law, Oxford University Press (Oxford, 2020), p 265

Inherently indiscriminate weapons

The first general principle, which seeks to protect civilians and is a logical corollary of the prohibition on indiscriminate attacks, makes it unlawful to use inherently indiscriminate weapons.2ICRC, Customary IHL Rule 71: ‘Weapons That Are by Nature Indiscriminate’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule71

An indiscriminate weapon is one that either cannot be targeted at a military objective or whose effects cannot be sufficiently limited.3 Art 51(4)(b)–(c); commentary to ICRC, Customary IHL Rule 71: ‘Weapons That Are by Nature Indiscriminate’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule71 A few weapons are prohibited as being inherently indiscriminate. This would be the case for biological weapons4 See, eg, UN General Assembly Resolution 2603 A (XXIV), 16 December 1969, second preambular para; commentary to ICRC, Customary IHL Rule 71: ‘Weapons That Are by Nature Indiscriminate’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule71 and certain unguided missiles,5 See ICRC, Commentary on the Additional Protocols of 8 June 1977 to the Geneva Conventions of 12 August 1949, para 1958 but not necessarily for anti-personnel mines, which may be used in a discriminate manner (for instance, by placing them in a marked and fenced area, especially where military personnel patrol the exterior of it).6 A. Bellal and S. Casey-Maslen, The Additional Protocols to the Geneva Conventions in Context, Oxford University Press (Oxford, 2022), para 12.43

Superfluous injury and unnecessary suffering

The second general principle, which primarily protects combatants and others who are directly participating in hostilities, prohibits the use in any armed conflict of means and methods of warfare that are of a nature to cause superfluous injury or unnecessary suffering.7 Art 23(e), 1899 and 1907 Hague Regulations; Art 35(2) AP I; ICRC, Customary IHL Rule 70: ‘Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule70 Already codified in the 1868 St Petersburg Declaration, the rule prohibits the use of weapons whose characteristics are such that they would inflict injury or suffering beyond what is necessary to render a fighter hors de combat.8 The International Court of Justice (ICJ) has defined unnecessary suffering as ‘a harm greater than that unavoidable to achieve legitimate military objectives’: ICJ, The Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 8 July 1996, para 78

The use of the following weapons in any armed conflict would be unlawful under the unnecessary suffering rule: explosive bullets; projectiles filled with glass; bullets that easily expand or flatten in the human body; poison and poisoned weapons; weapons containing any substance intended to aggravate a wound; bayonets with a serrated edge; and lances with barbed heads.9 ICRC, Commentary on the Additional Protocols of 8 June 1977 to the Geneva Conventions of 12 August 1949, para 1419

Rules on specific weapons

In addition to the general principles on means and methods of warfare outlined above, certain IHL rules address specific weapons. By way of example, customary IHL (and the corresponding underlying treaties) prohibits the use of, among others, chemical weapons10 ICRC, Customary IHL Rule 74: ‘Chemical Weapons’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule74; 1993 Chemical Weapons Convention and blinding laser weapons,11 ICRC, Customary IHL Rule 86: ‘Blinding Laser Weapons’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule86; 1995 Protocol IV on Blinding Laser Weapons to the 1980 Convention on Certain Conventional Weapons and limits to specific circumstances the use of riot control agents12 ICRC, Customary IHL Rule 75: ‘Riot Control Agents’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule75; Art I(5), 1993 Chemical Weapons Convention and of incendiary weapons.13 ICRC, Customary IHL Rules 84–85; A. Bellal and S. Casey-Maslen, The Additional Protocols to the Geneva Conventions in Context, Oxford University Press (Oxford, 2022), para 12.19; United Nations Office for Disarmament Affairs(UNODA), The Regulation of Incendiary Weapons under International Humanitarian Law, Geneva, 2024, paras 4.2 and 6.2 Other, more recent treaties prohibit or restrict the use of such weapons as cluster munitions14 Art 1, 2008 Convention on Cluster Munitions and nuclear weapons.15 Art 1, 2017 Treaty on the Prohibition of Nuclear Weapons

Rules on Methods of Warfare

Certain methods of warfare are prohibited in any circumstances, such as an order not to accept the surrender of the enemy (“denial of quarter”), perfidy and pillage. Others are allowed but only under strictly defined circumstances, such as the seizure and destruction of property.

Prohibition on perfidy

The prohibition on perfidy is a long-standing customary rule of IHL, applicable in all armed conflicts and designed to outlaw conducts that would undermine the protections afforded by IHL to certain objects and individuals.16 ICRC, Customary IHL Rule 65: ‘Perfidy’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule65 The rule prohibits to act so as to cause an adversary to believe that a person or object is entitled to protection under the rules of IHL, with the intent to betray that confidence in order to kill, injure or capture that adversary.17 Art 37(1) AP I; ICRC, Customary IHL Rule 65: ‘Perfidy’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule65 Thus, pretending to be medical personnel engaged in treating patients and then attacking the enemy would be unlawful. Conversely, ruses of war (ie, tactics intended to mislead an adversary or to induce them to act recklessly based upon an erroneous representation of facts that would not, if true, imply protection under IHL) are not prohibited.18 Art 37(2) AP I; ICRC, Customary IHL Rule 57: ‘Ruses of War’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule57

Giving or ordering no quarter

The prohibition ‘to order that there shall be no survivors, to threaten an adversary therewith or to conduct hostilities on this basis’19  Art 40 AP I ICRC, Customary IHL Rule 46: ‘Orders or Threats that No Quarter Will Be Given’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule46. See also Art 4(1) AP II is linked to the protection of the persons hors de combat, who may no longer be attacked as long as thy do not engage in hostile acts or attempt escape.20 Art 41(1) AP I; ICRC, Customary IHL Rule 47 : ‘Attacks against Persons Hors de Combat’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule47. Persons hors de combat include persons that are in the power of the adverse party or who clearly express an intention to surrender.21 Art 41(2) AP I; ICRC, Customary IHL Rule 47 : ‘Attacks against Persons Hors de Combat’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule47.

Treatment of property

The overarching rule in respect of property is the absolute prohibition on pillage.22 Arts 28 and 47, 1899 and 1907 Hague Regulations; Art 33(2) GC IV; Art 4(2)(g) AP II; ICRC, Customary IHL Rule 52: ‘Pillage’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule52 Described by the ICRC as ‘a specific application of the general principle of law prohibiting theft’,23 ICRC, Commentary to Customary IHL Rule 52 pillage is defined in the ICC’s Elements of Crimes as an appropriation of property, without the consent of the owner, for private or personal use.24 ICC, Elements of Crimes, ‘War crime of pillaging – ICC Statute, Arts 8(2)(b)(xvi) and 8(2)(e)(v), pp 17 and 25

In addition to this key provision, IHL rules governing belligerent occupation (largely codified in the Hague Regulations of 1899 and 1907 and in GC IV, and reflective of customary law) provide a detailed regulation for the treatment of property in occupied territory; this set of rules distinguishes between appropriation and destruction of property, and between private and public, movable and immovable property. Thus, IHL prohibits as a general rule the confiscation of private property; contributions in kind and requisition of essential supplies may exceptionally occur to satisfy the needs of the occupying forces in cases of imperative military necessity, if the requirements of the civilian population have been taken into account and subject to the payment of fair value.25 Arts 46 and 52, 1899 and 1907 Hague Regulations; Art 55(2) GC IV; ICRC, Customary IHL Rule 51: ‘Public and Private Property in Occupied Territory’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule51 When it comes to public property, movable objects (except for the property of municipalities and of institutions dedicated to religion, charity and education, the arts and sciences) may be confiscated (without compensation) and used for military operations.26 Arts 53 and 56, 1899 and 1907 Hague Regulations; ICRC, Customary IHL Rule 40: ‘Respect for Cultural Property’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule40; ICRC, Customary IHL Rule 51: ‘Public and Private Property in Occupied Territory’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule51 The same applies to war booty, ie military equipment captured or found on the battlefield: see ICRC, Customary IHL Rule 49: ‘War Booty’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule49 On the other hand, immovable public property must be administered by the Occupying Power according to the rules of usufruct, safeguarding its underlying value.27 Art 55, 1899 and 1907 Hague Regulations; ICRC, Customary IHL Rule 51: ‘Public and Private Property in Occupied Territory’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule51 Finally, IHL only allows for the destruction of the public and private property in occupied territory when this is rendered absolutely necessary by military operations.28 Art 53, GC IV; ICRC, Customary IHL Rule 51: ‘Public and Private Property in Occupied Territory’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule51.

The IHL rules governing seizure and destruction of property in occupied territory are more restrictive than those governing non-occupied territory and situations of NIAC. In these latter scenarios, the prohibition on seizing and destroying the property of an adversary can be forfeited when ‘imperatively demanded by the necessities of war’.29 Art 23(g), 1899 and 1907 Hague Regulations; ICRC, Customary IHL Rule 50: ‘Destruction and Seizure of Property of an Adversary’, https://ihl-databases.icrc.org/en/customary-ihl/v1/rule50; ICRC, Updated Commentary on GC IV, 2025, para 3456