Related Rule
United States of America
Practice Relating to Rule 71. Weapons That Are by Nature Indiscriminate
The US Air Force Pamphlet (1976) states:
The existing law of armed conflict does not prohibit the use of weapons whose destructive force cannot strictly be confined to the specific military objective. Weapons are not unlawful simply because their use may cause incidental casualties to civilians and destruction of civilian objects. Nevertheless, particular weapons or methods of warfare may be prohibited because of their indiscriminate effects … Indiscriminate weapons are those incapable of being controlled, through design or function, and thus they can not, with any degree of certainty, be directed at military objectives. For example, in World War II German V-1 rockets, with extremely primitive guidance systems yet generally directed toward civilian populations, and Japanese incendiary balloons without any guidance systems were regarded as unlawful. Both weapons were, as deployed, incapable of being aimed specifically at military objectives. Use of such essentially unguided weapons could be expected to cause unlawful excessive injury to civilians and damage to civilian objects … Some weapons, though capable of being directed only at military objectives, may have otherwise uncontrollable effects so as to cause disproportionate civilian injuries or damage. Biological warfare is a universally agreed illustration of such an indiscriminate weapon. Uncontrollable effects, in this context, may include injury to the civilian population of other states as well as injury to an enemy’s civilian population. Uncontrollable refers to effects which escape in time or space from the control of the user as to necessarily create risks to civilian persons or objects excessive in relation to the military advantage anticipated. International law does not require that a weapon’s effects be strictly confined to the military objectives against which it is directed, but it does restrict weapons whose foreseeable effects result in unlawful disproportionate injury to civilians or damage to civilian objects. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-3(c).
As regards new weapons, the Pamphlet states:
A new weapon or method of warfare may be illegal, per se, if it is restricted by international law including treaty or international custom … [T]he legality of new weapons … is determined by whether the weapon’s … effects are indiscriminate as to cause disproportionate civilian injury or damage to civilian objects. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 6-7(a).
The US Air Force Commander’s Handbook (1980) states:
Weapons that are incapable of being controlled enough to direct them against a military objective … are forbidden. A weapon is not unlawful simply because its use may cause incidental or collateral casualties to civilians, as long as those casualties are not foreseeably excessive in light of the expected military advantage. Using unpowered and uncontrolled balloons to carry bombs is thus forbidden, since these weapons would be incapable of being directed against a military objective. 
United States, Air Force Pamphlet 110-34, Commander’s Handbook on the Law of Armed Conflict, Judge Advocate General, US Department of the Air Force, 25 July 1980, § 6-2(b).
The US Naval Handbook (1995) states: “Weapons which by their nature are incapable of being directed specifically against military objectives, and therefore that put noncombatants at equivalent risk, are forbidden due to their indiscriminate effect.” 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 9.1.
The Handbook further specifies:
Weapons that are incapable of being controlled (i.e., directed at a military target) are forbidden as being indiscriminate in their effect. Drifting armed contact mines and long-range unguided missiles (such as the German V-1 and V-2 rockets of World War II) fall into this category. A weapon is not indiscriminate simply because it may cause incidental or collateral civilian casualties, provided such casualties are not foreseeably excessive in light of the expected military advantage to be gained. An artillery round that is capable of being directed with a reasonable degree of accuracy at a military target is not an indiscriminate weapon simply because it may miss its mark or inflict collateral damage. Conversely, uncontrolled balloon-borne bombs, such as those released by the Japanese against the west coast of the United States and Canada in World War II, lack that capability of direction and are, therefore, unlawful. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-2.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Transportation, US Coast Guard, October 1995 (formerly NWP 9 (Rev. A)/FMFM 1-10, October 1989), § 9.1.2.
The US Naval Handbook (2007) states that “weapons, which by their nature are incapable of being directed specifically against military objectives, and therefore that put civilians and noncombatants at equivalent risk, are forbidden due to their indiscriminate effect”. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 9.1.
The Handbook also states:
Weapons that are incapable of being directed at a military objective are forbidden as being indiscriminate in their effect. Drifting armed contact mines and long-range unguided missiles (such as the German V-1 and V-2 rockets of World War II) fall into this category. A weapon is not indiscriminate simply because it may cause incidental or collateral civilian casualties, provided such casualties are not foreseeably excessive in light of the anticipated military advantage to be gained. An artillery round that is capable of being directed with a reasonable degree of accuracy at a military target is not an indiscriminate weapon simply because it may miss its mark or inflict collateral damage. Conversely, uncontrolled balloon-borne bombs, such as those released by the Japanese against the west coast of the United States and Canada in World War II, lack that capability of direction and are, therefore, unlawful. 
United States, The Commander’s Handbook on the Law of Naval Operations, NWP 1-14M/MCWP 5-12.1/COMDTPUB P5800.7, issued by the Department of the Navy, Office of the Chief of Naval Operations and Headquarters, US Marine Corps, and Department of Homeland Security, US Coast Guard, July 2007, § 9.1.2.
In 1972, the General Counsel of the US Department of Defense stated:
Existing laws of armed conflict do not prohibit the use of weapons whose destructive force cannot be limited to a specific military objective. The use of such weapons is not proscribed when their use is necessarily required against a military target of sufficient importance to outweigh inevitable, but regrettable, incidental casualties to civilians and destruction of civilian objects … I would like to reiterate that it is recognized by all states that they may not lawfully use their weapons against civilian population[s] or civilians as such, but there is no rule of international law that restrains them from using weapons against enemy armed forces or military targets. The correct rule of international law which has applied in the past and continued to apply to the conduct of our military operations in Southeast Asia is that “the loss of life and damage to property must not be out of proportion to the military advantage to be gained”. 
United States, Letter from J. Fred Buzhardt, General Counsel of the Department of Defense, to Senator Edward Kennedy, Chairman of the Subcommittee on Refugees of the Committee on the Judiciary, 22 September 1972, AJIL, Vol. 67, 1973, p. 124.
In 1987, during the debate on Security Council Resolution 598 concerning the use of chemical weapons in the Iran–Iraq war, the United States stated that chemical weapons “honored no distinction between combatants and non-combatants”. 
United States, Statement before the UN Security Council, UN Doc. S/PV.2750, 20 July 1987, p. 18.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense accused Iraq of “indiscriminate Scud missile attacks”. 
United States, Department of Defense, Final Report to Congress on the Conduct of the Persian Gulf War, 10 April 1992, Appendix O, The Role of the Law of War, ILM, Vol. 31, 1992, p. 635.
In 1992, in a review of the legality of extended range anti-armour munition, the US Department of the Air Force stated:
International law also forbids the use of weapons or means of warfare which are “indiscriminate.” A weapon is indiscriminate if it cannot be directed at a military objective or if, under the circumstances, it produces excessive civilian casualties in relation to the concrete and direct military advantage anticipated. The ERAM [extended range antiarmor munition] is clearly capable of being directed at a military objective, i.e., enemy armor formations.  
United States, Department of the Air Force, Judge Advocate General, Legal Review: Extended Range Antiarmor Munition (ERAM), 16 April 1992, § 4.
In 1993, in its report to Congress on the protection of natural and cultural resources during times of war, the US Department of Defense stated:
Finally, with the poor track record of compliance with the law of war by some nations, the United States has a responsibility to protect against threats that may inflict serious collateral damage to our own interests and allies. These threats can arise from any nation that does not have the capability or desire to respect the law of war. One example is Iraq’s indiscriminate use of SCUDs during the Iran–Iraq War and the Gulf War. These highly inaccurate theater ballistic missiles can cause extensive collateral damage well out of proportion to military results. 
United States, Department of Defense, Report to Congress on International Policies and Procedures Regarding the Protection of Natural and Cultural Resources During Times of War, 19 January 1993, p. 203.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United States stated:
It has been argued that nuclear weapons are unlawful because they cannot be directed at a military objective. This argument ignores the ability of modern delivery systems to target specific military objectives with nuclear weapons, and the ability of modern weapon designers to tailor the effects of a nuclear weapon to deal with various types of military objectives. Since nuclear weapons can be directed at a military objective, they can be used in a discriminate manner and are not inherently indiscriminate. 
United States, Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 23; see also Oral pleadings before the ICJ, Nuclear Weapons case, 15 November 1995, Verbatim Record CR 95/34, p. 70.
In 1998, in a legal review of Oleoresin Capsicum (OC) pepper spray, the Deputy Assistant Judge Advocate General of the US Department of the Navy stated:
A weapon must be discriminating, or capable of being controlled (i.e., it can be directed against intended targets). Those weapons which cannot be employed in a manner which distinguishes between lawful combatants and noncombatants violate these principles. Indiscriminate weapons are prohibited by customary international law and treaty law.
The OC system contemplated for acquisition and employment by the Marine Corps is specifically designed to limit its effects only to intended targets. The contemplated OC dispersers utilize a target specific stream of ballistic droplets for controlled delivery and minimal cross contamination (i.e., point target delivery), rather than an aerosolized spray which increases the likelihood of unintended subject impact. Provided the weapon is employed in a discriminating manner, the principle of distinction/discrimination presents no prohibition to acquisition and employment of OC in appropriate circumstances. 
United States, Department of the Navy, Deputy Assistant Judge Advocate General, International and Operational Law Division, Legal Review of Oleoresin Capsicum (OC) Pepper Spray, 19 May 1998, § 5.
The Report on US Practice states: “It is the opinio juris of the United States that customary international law prohibits the use of indiscriminate weapons. Indiscriminate weapons are those that cannot be directed at a military objective.” 
Report on US Practice, 1997, Chapter 3.3.