United States, Military Manual (Land Warfare) (2019)

Ensuring the Legality of Weapons and Weapon Systems

2-200. Under longstanding DOD policies, there are requirements to review the legality of the acquisition or procurement of weapons. Although the United States is not a party to Additional Protocol I, many allies and potential multinational partners of the United States are required to determine the legality of new weapons as a treaty obligation under article 36 of Additional Protocol I.

2-201. Prior to fielding or deploying any weapon and weapon system, including non-lethal weapons, cyber weapons and cyber weapon systems, DOD requires the legal review of the acquisition or procurement be reviewed to ensure compliance with all applicable U.S. domestic law and international legal obligations of the United States, whether derived from LOAC, international law, international agreements, customary international law. (See DODD 5000.01, ¶E1.1.15). To implement this policy, all weapons, weapon systems, and munitions must receive a legal review by an attorney authorized by the Military Department. For the Marine Corps, the Judge Advocate General of the Navy (Code 10) has been authorized to conduct the legal review (see SECNAVINST 5000.2E) and OTJAG-NSL has been authorized to conduct legal reviews for the Army (see AR 27-53).

Note: The general legality of a weapon, weapon system, or munition discussed in this manual does not obviate the requirement for a legal review of the acquisition or procurement of each new weapon, weapon system, or munition.

2-202. A review of the legality of a weapon system, weapon or munition under LOAC should address each of the following questions: whether a specific rule of law prohibits or restricts its use, whether the weapon is calculated to cause unnecessary suffering or superfluous injury, and whether the weapon is inherently indiscriminate.

Specific Rule of Law

2-203. Whether a specific rule of law—such as a U.S. treaty obligation, a rule accepted by the United States as customary international law, or a domestic law—prohibits or restricts the use of the weapon in question is first considered. The use of the following types of weapons is prohibited:

  • poison, poisoned weapons, poisonous gases, and other chemical weapons;
  • biological weapons;
  • certain environmental modification techniques;
  • weapons that injure by fragments that are non-detectable by X-rays;
  • certain types of mines, booby-traps, and other devices; and
  • lasers specifically designed to cause permanent blindness to unenhanced vision.

For discussion of these weapons, refer to Chapter VI of the DOD Law of War Manual.

2-204. Certain types of weapons are subject to specific rules that apply to their use by the U.S. armed forces. These rules may reflect U.S. obligations under international law or national policy. These weapons include:

  • mines, booby-traps, and other devices (except certain specific classes of prohibited mines, booby-traps, and other devices);
  • cluster munitions;
  • incendiary weapons;
  • laser weapons (except blinding lasers);
  • riot control agents;
  • herbicides;
  • nuclear weapons;
  • explosive ordnance.

For more information, consult the DOD Law of War Manual and specific policies and guidance applicable to particular types of weapons. It may be appropriate in the review of such weapons to advise on these types of restrictions or obligations.

2-205. If there is no specific prohibition, the following questions are considered:

  • Whether the weapon is calculated to cause superfluous injury, in violation of the Hague Regulations Article 23(e); and
  • Whether the weapon is inherently indiscriminate.

Unnecessary Suffering/Superfluous Injury

2-206. A weapon review addresses whether the weapon is calculated to cause unnecessary suffering or superfluous injury in violation of the standard stated in Hague Regulations Article 23(e). The terms “unnecessary suffering” and “superfluous injury” are synonymous in the context of this analysis. Superfluous injury generally is determined in light of the practice of nations and in evaluation of a specific weapon. Superfluous injury is assessed in the sense of the design of a particular weapon or its employment, and not in terms of how a person affected by the weapon would be subjectively affected by it. Use of “calculated to cause” in the Hague Regulations Article 23(e) helps convey that the legal standard is focused on assessing the intended purpose or purposes of the weapon’s developer in developing the weapon.

2-207. The prohibition of weapons calculated to cause superfluous injury or unnecessary suffering constitutes acknowledgement that the use of weapons in war causes suffering, including injury and loss of life. A weapon cannot be declared unlawful merely because it may cause severe injury or suffering. Nor is a lawyer reviewing the legality of a weapon required to foresee or anticipate all possible uses or misuses of a weapon, for almost any weapon can be misused in ways that might be prohibited.

2-208. LOAC has no internationally agreed upon definition for superfluous injury. The determination is whether a weapon’s employment for its normal or expected use would be prohibited under some or all circumstances. A weapon would be deemed to cause superfluous injury only if it inevitably, or in its normal use, has a particular effect—and the injury caused as a result of this use is considered by governments as manifestly disproportionate to the military necessity for it, that is, to the military advantage to be gained from its employment. This determination cannot be made in isolation from other weapons. A weapon’s effects must be weighed in light of comparable, lawful weapons in use on the modern battlefield.

2-209. Importantly, the effect of the use of a weapon in combat is not the sole criterion for determining whether a weapon is calculated to cause superfluous injury; effects will differ widely as a result of the constantly shifting nature of the battlefield. For example, a weapon that can incapacitate or wound lethally at 300 meters or longer ranges may result in a greater degree of incapacitation or greater lethality when used against targets at lesser ranges. Similarly, the necessity for the use of a weapon of sufficient power to destroy materiel or, for example, a reinforced object such as a tank, a bunker, or an aircraft hanger may have a devastating effect on enemy military personnel in, on, or adjacent to it at the time of its attack, or on enemy military personnel struck directly by a weapon intended for a vehicle or entrenched defensive position. In both cases, the use of these weapons would not be unlawful.

2-210. An objective in war is to impose destruction upon the military forces of the enemy and other military objectives that contribute to its ability to wage war. The prohibition of weapons calculated to cause superfluous injury is not intended to limit the legitimate pursuit of those objectives through the employment of lawful weapons against lawful targets. This principle does not prohibit bringing a preponderance of force on an opposing military force in order to subdue or destroy it.

2-211. In summary, in determining whether a weapon causes superfluous injury, a legal review includes all of the following:

  • An evaluation of the military necessity of the weapon;
  • An analysis of the intended results of its use (including design, that is, what it is designed to do, and intended employment, that is, how it is intended to be used in combat); and
  • A comparison of the weapon with lawful comparable weapons already in use.

Inherently Indiscriminate Weapons

2-212. A fundamental principle of LOAC is that in the application of force, a nation’s military forces must direct their attacks against enemy military objectives and, furthermore, must distinguish these military objectives from the civilian population, individual civilians not taking a direct part in hostilities, and civilian objects. Only combatants, civilians taking a direct part in hostilities, and other military objectives may be targeted.

2-213. The principle of distinction in targeting requires that the weapon be capable of being directed against a military objective. “Capable of being directed” does not require terminal guidance to the target or use of the most precise weapon under all circumstances. It does, however, prohibit “blind weapons.” Such weapons cannot, with any reasonable assurance, be directed against a military objective. Indiscriminate weapons also include those that are essentially random in their effects and would be as likely to hit civilians as combatants.

2-214. Soldiers and Marines may presume a weapon to be lawful, that is, consistent with LOAC and arms control obligations of the United States, if it has received the requisite legal review and has been issued in accordance with relevant regulations of the military service concerned. Questions regarding the legality of a particular weapon, weapon system, or munition should be addressed to the appropriate legal officer responsible for the legal review as identified in paragraph 2-202.

2-215. The application of LOAC related to specific targeting issues will often be addressed at the time of employment, to be determined by the on-scene commander under the circumstances ruling at the time. Such issues do not determine the lawfulness of the weapon, weapon system, or munition provided for in the legal review (see DOD Law of War Manual, 6.1.1). The commander authorizing a weapon’s use must consider its characteristics, whether civilians are present, and other factors in order to ensure consistency with mission Rules of Engagement and LOAC proscriptions on directing attacks at civilians not taking a direct part in hostilities or who otherwise do not pose a threat to U.S. forces

US Department of the Army and US Marine Corps, FM 6-27/MCTP 11-10C: The Commander’s Handbook on the Law of Land Warfare (August 2019) 2-32–2-34