Related Rule
United States of America
Practice Relating to Rule 12. Definition of Indiscriminate Attacks
Section B. Attacks which cannot be directed at a specific military objective
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. 
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).
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. 
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 … 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. 
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 indiscriminate attacks include:
attacks that employ a method or means of combat that cannot be directed at a specific military objective (e.g., declaring an entire city a single military objective and attacking it by bombardment when there are actually several distinct military objectives throughout the city that could be targeted separately). 
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, § 5.3.2.
In 1992, a legal review by the US Department of the Air Force of the legality of extended range anti-armour munition 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. 
United States, Department of the Air Force, The Judge Advocate General, Legal Review: Extended Range Antiarmor Munition (ERAM), 16 April 1992, § 4.
Prior to the adoption of UN General Assembly Resolution 47/37 in 1992 on the protection of the environment in times of armed conflict, Jordan and the United States submitted a memorandum to the Sixth Committee of the UN General Assembly entitled “International Law Providing Protection to the Environment in Times of Armed Conflict”, which provided:
It is a war crime to employ acts of violence not directed at specific military objectives, to employ a method or means of combat which cannot be directed at a specific military objective, or to employ a means or method of combat the effects of which cannot be limited as required by the law of armed conflict. 
Jordan and United States, International Law Providing Protection to the Environment in Times of Armed Conflict, annexed to Letter dated 28 September 1992 to the Chairman of the Sixth Committee of the UN General Assembly, UN Doc. A/C.6/47/3, 28 September 1992, § 1(g).
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 (WHO) case in 1994, the United States stated that “it is unlawful to conduct any indiscriminate attack, including those employing weapons that … cannot be directed at a military objective”. 
United States, Written statement submitted to the ICJ, Nuclear Weapons (WHO) case, 10 June 1994, p. 27; Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 23.
In submitting the 1996 Amended Protocol II to the Convention on Certain Conventional Weapons to Congress for advice and consent to ratification, the US President stated that the prohibition of indiscriminate use of mines, booby-traps and other devices as defined in Article 3(8)(b) of the Protocol “is already a feature of customary international law that is applicable to all weapons”. 
United States, Message from the US President transmitting Protocols II, III and IV to the Convention on Certain Conventional Weapons to the Senate for consent to ratification, Treaty Doc. 105-1, Washington, D.C., 7 January 1997, Tab (A), p. 13.
In 1998, in a legal review of Oleoresin Capsicum (OC) pepper spray in 1998, 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. 
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.
According to the Report on US Practice, it is the opinio juris of the United States that indiscriminate attacks include attacks that employ methods or means of warfare that cannot be directed at a military objective. 
Report on US Practice, 1997, Chapter 1.4.