United States, Military Manual (2015)

6.2 DoD Policy of Reviewing the Legality of Weapons

As provided in DoD issuances, DoD policy for many years has required the legal review of the intended acquisition or procurement of weapons or weapon systems; this review includes ensuring that such acquisition or procurement is consistent with the law of war.7 These DoD policy requirements have been implemented in Military Department regulations.8

The fact that the legality of a weapon is discussed in this manual does not obviate the requirement under applicable directives or regulations for a legal review of a weapon or weapon system that is to be acquired or procured.

6.2.1 Review of New Types of Weapons

The development of new types of weapons has often resulted in public denunciation of their allegedly cruel effects and in attempts to prohibit their use in armed conflict.9 This has been true of the crossbow, siege engines for hurling projectiles, firearms, gunpowder, bayonets, and other weapons that have since been widely recognized as not prohibited by the law of war.

Like other aspects of the law of war, the rules relating to weapons are generally characterized as prohibitive law forbidding certain weapons or the use of weapons in certain instances rather than positive law authorizing the weapon or its use.10 The lawfulness of the use of a type of weapon does not depend on the presence or absence of authorization, but, on the contrary, on whether the weapon is prohibited.11 Thus, the mere fact that a weapon is novel or employs new technology does not mean that the weapon is illegal.12 The law of war does not require States to establish a general practice of using a weapon before it is to be regarded as legal. Moreover, it would appear absurd to suggest that a new type of weapon should automatically be prohibited because there is no State practice supporting such use, or to suggest that States must continue using a weapon in each conflict simply to maintain its legality.

The review of the acquisition or procurement of a weapon for consistency with U.S. law of war obligations should consider three questions to determine whether the weapon’s acquisition or procurement is prohibited:

  • whether the weapon’s intended use is calculated to cause superfluous injury;13
  • whether the weapon is inherently indiscriminate;14 and
  • whether the weapon falls within a class of weapons that has been specifically prohibited.15

If the weapon is not prohibited, the review should also consider whether there are legal restrictions on the weapon’s use that are specific to that type of weapon.16 If any specific restrictions apply, then the intended concept of employment of the weapon should be reviewed for consistency with those restrictions.

Lastly, it may be appropriate to advise whether other measures should be taken that would assist in ensuring compliance with law of war obligations related to the type of weapon being acquired or procured. For example, it may be appropriate to advise on the need for training programs and other practical measures, such as promulgating doctrine and rules of engagement related to that type of weapon.17

Article 36 of AP I provides:

In the study, development, acquisition or adoption of a new weapon, means or method of warfare, a High Contracting Party is under an obligation to determine whether its employment would, in some or all circumstances, be prohibited by this Protocol or by any other rule of international law applicable to the High Contracting Party.

The DoD policy and practice of conducting legal reviews of weapons preceded this AP I provision.18

6.2.4 Policy and Processes for Review of Applicable Arms Control Obligations

DoD has had a separate, but complementary, policy and practice requiring review of its activities (e.g., research, development, and testing of weapons) to ensure that these activities are consistent with the arms control agreements to which the United States is a Party.19

7 For example, DOD DIRECTIVE 5000.01, The Defense Acquisition System, ¶E1.1.15 (May 12, 2003, certified current as of Nov. 20, 2007) (“The acquisition and procurement of DoD weapons and weapon systems shall be consistent with all applicable domestic law and treaties and international agreements (for arms control agreements, see DoD Directive 2060.1 (Reference (m), customary international law, and the law of armed conflict (also known as the laws and customs of war)[)]. An attorney authorized to conduct such legal reviews in the Department shall conduct the legal review of the intended acquisition of weapons or weapons systems.”); DOD DIRECTIVE 5000.01, The Defense Acquisition System, ¶4.2.10 (Mar. 15, 1996, cancelled by DoD Directive 5000.1 Oct. 23, 2000) (“DoD acquisition and procurement of weapons shall be consistent with applicable domestic law and all applicable treaties, customary international law, and the law of armed conflict (also known as the laws and customs of war).”); DOD INSTRUCTION 5500.15, Review of Legality of Weapons Under International Law, ¶II (Oct. 16, 1974, cancelled by DoD Instruction 5000.2 Feb. 23, 1991) (“All actions of the Department of Defense with respect to the acquisition and procurement of weapons, and their intended use in armed conflict, shall be consistent with the obligations assumed by the United States Government under all applicable treaties, with customary international law, and, in particular, with the laws of war.”).

8 For example, DEPARTMENT OF THE ARMY REGULATION 27-53, Review of Legality of Weapons Under International Law (Jan. 1, 1979); SECRETARY OF THE NAVY INSTRUCTION 5000.2E, Department of the Navy Implementation and Operation of the Defense Acquisition System and the Joint Capabilities Integration and Development System (Sept. 1, 2011); DEPARTMENT OF THE AIR FORCE INSTRUCTION 51-402, Legal Reviews of Weapons and Cyber Capabilities (Jul. 27, 2011).

9 1976 AIR FORCE PAMPHLET 110-31 ¶6-7a (“The development of new weapons or methods of warfare has often resulted in public denunciation of their allegedly cruel effects, and attempts to prohibit their use in warfare. This has been true of the crossbow, siege engines for hurling projectiles, firearms, gunpowder, bayonets and other less efficient methods of warfare.”).

10 Refer to § 1.3.3.1 (Law of War as Prohibitive Law).

11 Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 I.C.J. 226, 247 (¶52) (“Nor, however, is there any principle or rule of international law which would make the legality of the threat or use of nuclear weapons or of any other weapons dependent on a specific authorization. State practice shows that the illegality of the use of certain weapons as such does not result from an absence of authorization but, on the contrary, is formulated in terms of prohibition.”).

12 1976 AIR FORCE PAMPHLET 110-31 ¶6-7a (“A weapon or method of warfare may not be considered illegal solely because it is new or has not previously been used in warfare.”).

13 Refer to § 6.6 (Weapons Calculated to Cause Superfluous Injury).

14 Refer to § 6.7 (Inherently Indiscriminate Weapons).

15 Refer to § 6.4.2 (Specifically Prohibited Types of Weapons).

16 Refer to § 6.5.1 (Certain Types of Weapons With Specific Rules on Use).

17 Refer to § 6.15.2 (Feasible Precautions in the Employment of Laser Systems to Avoid the Incident of Permanent Blindness).

18 For example, DOD INSTRUCTION 5500.15, Review of Legality of Weapons Under International Law, ¶II (Oct. 16, 1974, cancelled by DoD Instruction 5000.2 Feb. 23, 1991) (“All actions of the Department of Defense with respect to the acquisition and procurement of weapons, and their intended use in armed conflict, shall be consistent with the obligations assumed by the United States Government under all applicable treaties, with customary international law, and, in particular, with the laws of war.”).

19 DOD DIRECTIVE 2060.1, Implementation of, and Compliance with, Arms Control Agreements, ¶4.6.7 (Jan. 9, 2001, certified current Nov. 24, 2003) (“For specific DoD-planned activities, [Heads of DoD components are to] seek clearance from the USD(AT&L), through the appropriate CRG [Compliance Review Group], on a timely basis, before taking any action, including but not limited to research, tests, development, exercises and operations that reasonably raises an issue of DoD compliance with an arms control agreement. For other compliance issues requiring resolution (such as those arising from an on-site inspection), seek resolution from the USD(AT&L), through the appropriate CRG. When there is doubt whether clearance or resolution is necessary, it shall be sought. If the issue involves a DoD SAP [Special Access Program], contact the Director of the cognizant DoD SAP Central Office and the Chairman of the DoD SAP Senior Review Group (SRG). The SRG Chairman will effect coordination with the appropriate CRG.”).

Department of Defense, Law of War Manual (June 2015, updated July 2023)