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United States of America
Practice Relating to Rule 145. Reprisals
Section C. Proportionality of reprisals
The US Field Manual (1956) states:
Form of Reprisal. What kinds of acts should be resorted to as reprisals is a matter for consideration by the injured party. Acts done by way of reprisals must not, however, be excessive. They must bear reasonable relation to the degree of violation committed by the enemy. 
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, § 497(e).
The US Air Force Pamphlet (1976) states:
A reprisal must be proportional to the original violation. Although a reprisal need not conform in kind to the same type of acts complained of (bombardment for bombardment, weapon for weapon) it may not significantly exceed the adversary’s violation either in violence or effect. Effective but disproportionate reprisals cannot be justified by the argument that only an excessive response will forestall further transgressions. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 10-7(c)(6).
The US Naval Handbook (1995) provides: “To be valid, a reprisal action must conform to the following criteria: … 6. Each reprisal must be proportional to the original violation.” 
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), § 6.2.3.1.
The Annotated Supplement to the US Naval Handbook (1997) states:
This rule [that a reprisal must be proportional to the original violation] is not of strict equivalence because the reprisal will usually be somewhat greater that the initial violation that gave rise to it. However, care must be taken that the extent of the reprisal is measured by some degree of proportionality and not solely by effectiveness. Effective but disproportionate reprisals cannot be justified by the argument that only an excessive response will forestall a further transgression … The acts resorted to by way of reprisal need not conform in kind to those complained of by the injured belligerent. The reprisal action taken may be quite different from the original act which justified it, but should not be excessive or exceed the degree of harm required to deter the enemy from continuance of his initial unlawful conduct. 
United States, Annotated Supplement to the Commander’s Handbook on the Law of Naval Operations, prepared by the Oceans Law and Policy Department, Center for Naval Warfare Studies, Naval War College, Newport, Rhode Island, November 1997, § 6.2.3.1, footnote 43.
The US Naval Handbook (2007) states: “To be valid, a reprisal action must conform to the following criteria: … 6. Each reprisal must be proportional to the original violation.” 
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, § 6.2.4.1.
In its judgment in the List case (The Hostages Trial) in the late 1940s, the US Military Tribunal at Nuremberg stated: “It is a fundamental rule that a reprisal may not exceed the degree of the criminal act it is designed to correct. Where an excess is knowingly indulged, it in turn is criminal and may be punished.” 
United States, Military Tribunal at Nuremberg, List case (The Hostages Trial), Judgment, 8 July 1947–19 February 1948.
In 1980, in a footnote to a memorandum of law on the “Reported Use of Chemical Agents in Afghanistan, Laos, and Kampuchea”, a legal adviser of the US Department of State noted: “Reprisals are permitted under the laws of war … only in proportion to the original violations.” 
United States, Department of State, Memorandum of law by a Legal Adviser on the “Reported Use of Chemical Agents in Afghanistan, Laos, and Kampuchea”, 9 April 1980, reprinted in Marian Nash Leich, Digest of United States Practice in International Law, 1980, Department of State Publication 9610, Washington, D.C., December 1986, pp. 1034 and 1041, footnote 38.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United States stated:
Reprisals would be lawful if conducted in accordance with the applicable principles governing belligerent reprisals. Specifically … the reprisals must be proportionate to the violations [of the law of armed conflict by the enemy] … As in the case of other requirements of the law of armed conflict, a judgment about compliance of any use of nuclear weapons with these requirements would have to be made on the basis of the actual circumstances in each case, and could not be made in advance or in the abstract. 
United States, Written statement submitted to the ICJ, Nuclear Weapons case, 20 June 1995, p. 30.