Related Rule
United States of America
Practice Relating to Rule 14. Proportionality in Attack
The US Field Manual (1956) states, in the context of sieges and bombardments, that “loss of life and damage to property must not be out of proportion to the military advantage to be gained”. 
United States, Field Manual 27-10, The Law of Land Warfare, US Department of the Army, 18 July 1956, as modified by Change No. 1, 15 July 1976, § 41.
The US Air Force Pamphlet (1976) states:
Complementing the principle of necessity and implicitly contained within it is the principle of humanity which forbids the infliction of suffering, injury or destruction not actually necessary for the accomplishment of legitimate military purposes. This principle of humanity results in a specific prohibition against unnecessary suffering, a requirement of proportionality and a variety of more specific rules examined later. The principle of humanity also confirms the basic immunity of civilian populations and civilians from being objects of attack during armed conflict. This immunity of the civilian population does not preclude unavoidable incidental civilian casualties which may occur during the course of attacks against military objectives, and which are not excessive in relation to the concrete and direct military advantage anticipated. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 1-3(a).
The US Air Force Commander’s Handbook (1980) states that “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 Instructor’s Guide (1985) states:
In attacking a military target, the amount of suffering or destruction must be held to the minimum necessary to accomplish the mission. Any excessive destruction or suffering not required to accomplish the objective is illegal as a violation of the law of war. 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 6.
The US Naval Handbook (1995) states:
It is not unlawful to cause incidental injury to civilians or collateral damage to civilian objects, during an attack upon a legitimate military objective. Incidental injury or collateral damage must not, however, be excessive in light of the military advantage anticipated by the attack. 
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), § 8.1.2.1.
The manual further specifies that “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:
The principle of proportionality is directly linked to the principle of distinction. While distinction is concerned with focusing the scope and means of attack so as to cause the least amount of damage to protected persons and property, proportionality is concerned with weighing the military advantage one expects to gain against the unavoidable and incidental loss to civilians and civilian property that will result from the attack. The principle of proportionality requires the commander to conduct a balancing test to determine if the incidental injury, including death to civilians and damage to civilian objects, is excessive in relation to the concrete and direct military advantage expected to be gained. Note that the principle of proportionality under the law of armed conflict is different than the term proportionality as used in self-defense. 
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, § 5.3.3.
The Handbook also states:
It is not unlawful to cause incidental injury to civilians, or collateral damage to civilian objects, during an attack upon a legitimate military objective. The principle of proportionality requires that the anticipated incidental injury or collateral damage must not, however, be excessive in light of the military advantage expected to be gained. 
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, § 8.3.1.
The Handbook further states that “excessive collateral damage must be avoided to the extent possible and, consistent with mission accomplishment and the security of the force, unnecessary human suffering prevented”. 
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, § 8.1.
The Handbook also states:
A military objective within a city, town, or village may, however, be bombarded if required for the submission of the enemy with the minimum expenditure of time, life, and physical resources. The anticipated incidental injury to civilians, or collateral damage to civilian objects, must not be excessive in light of the military advantage anticipated by the attack. 
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, § 8.9.1.1.
The Agent Orange case before the US Eastern States District Court in 2005 involved a class action suit filed on behalf of various Vietnamese nationals and an organization, the Vietnamese Association for Victims of Agent Orange/Dioxin, against Dow Chemical and other US chemical manufacturers for harms allegedly done to them and their land through the United States’ use of Agent Orange and other herbicides during the Vietnam War from 1965 to 1971 and by the South Vietnamese Government’s subsequent use of such herbicides until 1975. In dismissing the claims, the Court found that, while recognizing the evolution of international law since 1975, the use of herbicides did not violate, at the time they were used, either customary or conventional international law binding on the United States. On the question of proportionality and the use of herbicides, the Court stated:
The concept of military necessity or proportionality is a well accepted international norm governing the conduct of war. The United States Army’s manual, The Law of Land Warfare, states the rule succinctly: “[L]oss of life and damage to property must not be out of proportion to the military advantage to be gained.” U.S. DEP’T OF THE ARMY, FIELD MANUAL NO. 27-10, THE LAW OF LAND WARFARE ¶ 41 (1956).
American courts are fully capable of applying the proportionality concept in civil litigations as demonstrated by their handling of comparative negligence, proximate cause and other sophisticated doctrines. Nevertheless, the criminal procedural aspects of proportionality, the inherently subjective judgments necessary to determine whether the concept applies, and the dearth of illustrative prosecutions, all demonstrate that federal courts should pause before recognizing a civil private cause of action under the ATS [Alien Tort Statute] on the theory that the United States may have properly used herbicides in some situations for legitimate military purposes, but that it used too much of them in too many places … It cannot be shown that the military in the field – or the executive and legislative branches at home – violated proportionality norms when using herbicides in the Vietnam War.
As plaintiffs argue, a consensus among nations may gradually solidify into recognized international law provided that such consensus is reflected in state practice accompanied by opinio juris. No such understanding or consensus existed with respect to herbicides prior to 1975 – possibly in part because of the proportionality problem.  
United States, Eastern States District Court (EDNY), Agent Orange case, Judgment, 28 March 2005, pp. 219–221.
In 1972, the General Counsel of the US Department of Defense stated:
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”. A review of the operating authorities and rules of engagements for all of our forces in Southeast Asia, in air as well as ground and sea operations, by my office reveals that not only are such operations in conformity with this basic rule, but that in addition, extensive constraints are imposed to avoid if at all possible the infliction of casualties on noncombatants and the destruction of property other than that related to the military operations in carrying out military objectives. 
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 1974, at the Lucerne Conference of Government Experts on Weapons which may Cause Unnecessary Suffering or have Indiscriminate Effects, the head of the US delegation stated:
The law of war also prohibits attacks which, though directed at lawful military targets, entail a high risk of incidental civilian casualties or damage to civilian objects which is disproportionate to the military advantage sought to be secured by the attack. 
United States, Statement of 25 September 1974 at the Conference of Government Experts on Weapons which may Cause Unnecessary Suffering or have Indiscriminate Effects, Lucerne, 24 September-18 October 1974, reprinted in Arthur W. Rovine, Digest of United States Practice in International Law, 1974, Department of State Publication 8809, Washington, D.C., 1975, p. 713.
In 1987, the Deputy Legal Adviser of the US Department of State affirmed: “We support the principle … that attacks not be carried out that would clearly result in collateral civilian casualties disproportionate to the expected military advantage.” 
United States, Remarks of Michael J. Matheson, Deputy Legal Adviser, US Department of State, The Sixth Annual American Red Cross-Washington College of Law Conference on International Humanitarian Law: A Workshop on Customary International Law and the 1977 Protocols Additional to the 1949 Geneva Conventions, American University Journal of International Law and Policy, Vol. 2, 1987, p. 426.
In 1991, in reaction to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army stated:
The concept of “incidental loss of life excessive in relation to the military advantage anticipated” generally is measured against an overall campaign. While it is difficult to weigh the possibility of collateral civilian casualties on a target-by-target basis, minimization of collateral civilian casualties is a continuing responsibility at all levels of the targeting process. Combat is a give-and-take between attacker and defender, and collateral civilian casualties are likely to occur notwithstanding the best efforts of either party. What is prohibited is wanton disregard for possible collateral civilian casualties. 
United States, Letter from the Department of the Army to the legal adviser of the US Army forces deployed in the Gulf region, 11 January 1991, § 8(F), Report on US Practice, 1997, Chapter 1.5.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated:
While the prohibition contained in Article 23(g) [of the Hague Regulations] generally refers to intentional destruction or injury, it also precludes collateral damage of civilian objects or injury to noncombatant civilians that is clearly disproportionate to the military advantage gained in the attack of military objectives, as discussed below. As previously indicated, Hague IV was found to be customary international law in the course of war crimes trials following World War II, and continues to be so regarded.
An uncodified but similar provision is the principle of proportionality. It prohibits military action in which the negative effects (such as collateral civilian casualties) clearly outweigh the military gain. This balancing may be done on a target-by-target basis, as frequently was the case during Operation Desert Storm, but also may be weighed in overall terms against campaign objectives. CENTCOM [Central Command] conducted its campaign with a focus on minimizing collateral civilian casualties and damage to civilian objects. Some targets were specifically avoided because the value of destruction of each target was outweighed by the potential risk to nearby civilians or, as in the case of certain archaeological and religious sites, to civilian objects. 
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. 622.
In 1992, a legal review by the US Department of the Air Force of the legality of extended range anti-armour munition stated that, while legal as such, this munition “should, however, only be used in concentrations of civilians if the military necessity for such use is great, and the expected collateral civilian casualties would not be excessive in relation to the expected military advantage”. 
United States, Department of the Air Force, The Judge Advocate General, Legal Review: Extended Range Antiarmor Munition (ERAM), 16 April 1992, § 9; see also §§ 4, 5 and 8.
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 that cultural property, civilian objects and natural resources are protected from:
collateral damage that is clearly disproportionate to the military advantage to be gained in the attack of military objectives. The law of war acknowledges the unfortunate inevitability of collateral damage when military objectives and civilian objects (including cultural property and natural resources) are commingled. 
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. 202.
In its written statement submitted to the ICJ in the Nuclear Weapons (WHO) case in 1994, the United States stated:
It is unlawful to carry out any attack that may reasonably be expected to cause collateral damage or injury to civilians or civilian objects that would be excessive in relation to the military advantage anticipated from the attack. Whether an attack with nuclear weapons would be disproportionate depends entirely on the circumstances, including the importance of destroying the objective, the character, size and likely effects of the device, and the magnitude of the risk to civilians. 
United States, Written statement submitted to the ICJ, Nuclear Weapons (WHO) case, 10 June 1994, p. 27; see also Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 23.
The Report on US Practice states: “United States practice recognizes the principle of proportionality as part of the customary law of non-nuclear war.” 
Report on US Practice, 1997, Chapter 1.5.
In 2005, the US Department of Justice submitted a Statement of Interest of the United States to the US District Court for the Eastern District of New York prior to that Court’s consideration of Agent Orange Product Liability Litigation (The Vietnam Association for Victims of Agent Orange/Dioxin, et al. v. Dow Chemical Company, et al). That statement reiterated the US position that no rule of international law barred the use of chemical herbicides in war generally nor barred the destruction of crops intended for use by enemy forces. With regard to the principle of proportionality in attack, it stated:
There are simply no established international law standards of the specificity required by Sosa Sosa v. Alvarez-Machain, US Supreme Court, 2004] for establishing a federal common law cause of action for unnecessary or disproportionate use of military force.
Indeed, the very nature of the principles defy specificity, for they require the balancing of competing considerations and are inherently imprecise. That is, the rules do not proscribe any particular conduct that is readily identifiable. Rather, they require consideration by combatant commanders of a variety of factors – unique to the context of any particular military action – that affect decisions on what means may be available to achieve the military objective and whether the harms that particular military actions might cause would be disproportionate to the advantages attained. In light of the balancing nature of the principles, the very same act that might be deemed both necessary and proportionate in one circumstance, might be deemed unnecessary or disproportionate in another. It is one thing, therefore, to recognize that these principles generally exist in customary international law. It is “harder,” if not impossible, to “say which policies cross [the] line with the certainty afforded by Blackstone’s three common law offenses” 124 S. Ct. at 2769.
A recent report by a Committee established to review the NATO bombing campaign in Yugoslavia provides a good illustration of the imprecise nature of the rule of proportionality:
The main problem with the principle of proportionality is not whether or not it exists but what it means and how it is to be applied. It is relatively simple to state that there must be an acceptable relation between the legitimate destructive effect and undesirable collateral effects. For example, bombing a refugee camp is obviously prohibited if its only military significance is that people in the camp are knitting socks for soldiers. Conversely, an air strike on an ammunition dump should not be prohibited merely because a farmer is plowing a field in the area. Unfortunately, most applications of the principle of proportionality are not quite so clear cut. It is much easier to formulate the principle of proportionality in general terms than it is to apply it to a particular set of circumstances because the comparison is often between unlike quantities and values. One cannot easily assess the value of innocent human lives as opposed to capturing a particular military objective.
Final Report to the Prosecutor by the Committee Established to Review the NATO Bombing Campaign Against the Federal Republic of Yugoslavia ¶ 48 …
Because any consensus regarding the necessity and proportionality principles necessarily exists solely “at a high level of generality,” id at 2768 n.27, and because the principles are not “defined with.. .specificity,” id at 2761, and clearly have “less definite content” than the 18th Century offenses discussed in Sosa. id. at 2765, the Court should not recognize a federal common law cause of action for alleged violations of the necessity and proportionality principles. See also Flores. 343 F.3d at 160–61. 
United States, Department of Justice, Statement of Interest of the United States submitted to the US District Court for the Eastern District of New York, In re: Agent Orange Product Liability Litigation (The Vietnam Association for Victims of Agent Orange/Dioxin, et al. v. Dow Chemical Company, et al.), 12 January 2005,§ II, pp. 34–36.
In March 2010, in a speech given at the Annual Meeting of the American Society of International Law, the US State Department’s Legal Adviser stated:
[T]his Administration has carefully reviewed the rules governing targeting operations to ensure that these operations are conducted consistently with law of war principles, including:
- Second, the principle of proportionality, which prohibits attacks that may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, that would be excessive in relation to the concrete and direct military advantage anticipated.
In U.S. operations against al-Qaeda and its associated forces – including lethal operations conducted with the use of unmanned aerial vehicles – great care is taken to adhere to these principles in both planning and execution, to ensure that … collateral damage is kept to a minimum. 
United States, “The Obama Administration and International Law”, speech given by the Legal Adviser of the US Department of State, at the Annual Meeting of the American Society of International Law, Washington DC, 25 March 2010.
[emphasis in original]
While the core challenges in the protection of civilians identified in the previous reports of the Secretary-General still need our sustained attention, the new report also identifies several protection policy priorities that need to be explored. In particular the following “emerging” issues would benefit from our attention, and the Group of Friends stands ready to act as a platform to advance them. …
… [O]n the issue of lethal autonomous weapons systems (LAWS), the Group is of the view that further discussions are needed and it welcomes the fact that the issue will be examined in Geneva in May 2014, in the framework of the CCW [Convention on Certain Conventional Weapons]. The Group hopes that such discussions will also examine the issue with due consideration to the protection of civilians as part of a comprehensive debate including legal, military operational, technological and ethical perspectives. In time discussion should focus on the relevance of such systems to the protection of civilians, in particular in the context of IHL and with regard to the principles of distinction, precaution and proportionality. 
Uruguay, Statement by the permanent representative of Switzerland during a UN Security Council open debate on the protection of civilians in armed conflict made on behalf of the Group of Friends on the Protection of Civilians in Armed Conflict, namely Australia, Austria, Belgium, Brazil, Canada, France, Germany, Italy, Japan, Liechtenstein, Norway, Portugal, Switzerland, the United Kingdom and Uruguay, 12 February 2014, p. 2.
The US Naval Handbook (1995) states that the term military advantage “refers to the advantage anticipated from the military operation of which the attack is a part, taken as a whole, and not from isolated or particular parts of that operation”. 
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), § 8.1.2.1.
The US Naval Handbook (2007) states: “Military advantage may involve a variety of considerations, including the security of the attacking force.” 
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, § 8.2.
At the CDDH, the United States stated that in its view the expression “military advantage anticipated” was intended to refer to “the advantage anticipated from the attack considered as a whole and not only from isolated or particular parts of the attack”. 
United States, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.42, 27 May 1977, p. 241.
In 1991, in reaction to an ICRC memorandum on the applicability of IHL in the Gulf region, the US Department of the Army pointed out:
The concept of “incidental loss of life excessive in relation to the military advantage anticipated” generally is measured against an overall campaign. While it is difficult to weigh the possibility of collateral civilian casualties on a target-by-target basis, minimization of collateral civilian casualties is a continuing responsibility at all levels of the targeting process. 
United States, Letter from the Department of the Army to the legal adviser of the US Army forces deployed in the Gulf region, 11 January 1991, § 8(F).
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated that the balancing of collateral damage against military gain “may be done on a target-by-target basis, as frequently was the case during Operation Desert Storm, but also may be weighed in overall terms against campaign objectives”. 
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. 622.
The Report on US Practice states:
United States practice recognizes the principle of proportionality as part of the customary law of non-nuclear war. In applying this principle, it is necessary to consider military advantage not only on an immediate or target-by-target basis, but also in light of the military objectives of an entire campaign or operation. 
Report on US Practice, 1997, Chapter 1.5.
The US Naval Handbook (1995) states: “The commander must determine whether incidental injuries and collateral damage would be excessive, on the basis of an honest and reasonable estimate of the facts available to him.” 
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), § 8.1.2.1.
The US Naval Handbook (2007) states that “the commander must determine whether the anticipated incidental injuries and collateral damage would be excessive, on the basis of an honest and reasonable estimate of the facts available to him”. 
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, § 8.3.1.
At the CDDH, the United States stated:
Commanders and others responsible for planning, deciding upon or executing attacks necessarily have to reach decisions on the basis of their assessment of the information from all sources which is available to them at the relevant time. 
United States, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.42, 27 May 1977, p. 241.