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United States of America
Practice Relating to Rule 145. Reprisals
Section B. Reasonable notice and measure of last resort
The US Field Manual (1956) states:
Priority to Other Remedies. Other measures of securing compliance with the law of war should normally be exhausted before resort is had to reprisals. This course should be pursued unless the safety of the troops requires immediate drastic action and the persons who actually committed the offences cannot be secured. Even when appeal to the enemy for redress has failed, it may be a matter of policy to consider, before resorting to reprisals, whether the opposing forces are not more likely to be influenced by a steady adherence to the law of war on the part of their adversary. 
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(b).
The manual adds: “Reprisals are never adopted merely for revenge, but only as an unavoidable last resort to induce the enemy to desist from unlawful practices.” 
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(d).
The US Air Force Pamphlet (1976), in explaining reprisals, states:
(a) The action is taken in the last resort, in order to prevent the adversary from behaving illegally in the future.
(c) In order to be considered a reprisal, an act must have the following characteristics when employed:
(3) There must be reasonable notice that reprisals will be taken. What degree of notice is required will depend upon the particular circumstances of each case. Notice is normally given after the violation but may, in appropriate circumstances, predate the violation. An example of notice is an appeal to the transgressor to cease its offending conduct and punish those responsible. Thus, such an appeal may serve both as a plea for compliance and a notice to the adversary that reprisals will be undertaken.
(4) Other reasonable means to secure compliance must be attempted. The victim of a violation in order to justify taking a reprisal must first exhaust other reasonable means of securing compliance. This may involve appeals or notice … Finally, even if an appeal or other methods fail, reprisals should not be undertaken automatically since there are various other factors governing their employment. 
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(a) and (c).
The US Air Force Commander’s Handbook (1980) states: “The taking of reprisals should be preceded by a request for redress of the wrong.” 
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, § 8-4(b)(2).
The US Naval Handbook (1995) provides:
To be valid, a reprisal action must conform to the following criteria:
3. When circumstances permit, reprisal must be preceded by a demand for redress by the enemy of his unlawful acts.
5. Reprisal must only be used as a last resort when other enforcement measures have failed or would be of no avail. 
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 US Naval Handbook (2007) states:
To be valid, a reprisal action must conform to the following criteria:
3. When circumstances permit, reprisal must be preceded by a demand for redress by the enemy of its unlawful acts.
5. Reprisal must only be used as a last resort when other enforcement measures have failed or would be of no avail. 
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, discussing the taking of hostages in occupied territories, noted: “The occupant is required to use every available method to secure order and tranquillity before resort may be had to the taking and execution of hostages.” However, the Tribunal had previously stated:
Where innocent individuals are seized and punished for a violation of the laws of war which has already occurred, no question of hostages is involved. It is nothing more than the infliction of a reprisal. Throughout the evidence in the present case, we find the term hostage applied where a reprisal only was involved. 
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 stated: “Reprisals are permitted under the laws of war … only after other means of achieving this objective [i.e. “the limited purpose of compelling the other belligerent to observe the laws of war”] have been exhausted (including diplomatic protest).” 
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 a 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 … other means of securing compliance [of the enemy with the law of armed conflict] should be exhausted … 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.