Related Rule
United States of America
Practice Relating to Rule 140. The Principle of Reciprocity
The US Air Force Pamphlet (1976) notes:
The most important relevant treaties, the 1949 Geneva Conventions for the Protection of War Victims, are not formally conditioned on reciprocity. Parties to each Convention “undertake to respect and ensure respect for the present Convention in all circumstances” under Article 1 common to the Conventions. The Vienna Convention On the Law of Treaties, Article 60(5), also recognizes that the general law on material breaches, as a basis for suspending the operation of treaties, does not apply to provisions protecting persons in treaties of a humanitarian character. Yet reciprocity is an implied condition in other rules and obligations including generally the law of armed conflict. It is moreover a critical factor in actual observance of the law of armed conflict. Reciprocity is also explicitly the basis for the doctrine of reprisals. Additionally, a few obligations, such as those contained in the 1925 Geneva Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous, or Other Gases, and of Bacteriological Methods of Warfare, are even formally conditioned on reciprocal adherence. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 10-1(b).
[emphasis in original]
The Pamphlet further states:
The UN Resolutions and the Geneva Conventions set forth standards regardless of whether observance is reciprocated. Hence, reciprocity is neither a formal condition precedent qualifying the obligation to observe the Conventions, nor does lack of reciprocity excuse failures to comply. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 11-5.
The US Naval Handbook (1995) states:
Some obligations under the law of armed conflict are reciprocal in that they are binding on the parties only so long as both sides continue to comply with them. A major violation by one side will release the other side from all further duty to abide by that obligation. The concept of reciprocity is not applicable to humanitarian rules of law that protect the victims of armed conflict, that is, those persons protected by the 1949 Geneva Conventions. The decision to consider the United States released from a particular obligation following a major violation by the enemy will be made by the [National Command Authorities]. 
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), § 6.2.4.
The US Naval Handbook (2007) states:
Some obligations under the law of armed conflict are reciprocal in that they are binding on the parties only so long as both sides continue to comply with them. A major violation by one side will release the other side from all further duty to abide by that obligation. The concept of reciprocity is not applicable to humanitarian rules that protect the victims of armed conflict, that is, those persons protected by the 1949 Geneva Conventions. The decision to consider the United States released from a particular obligation following a major violation by the enemy will be made by the president. 
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, p. 19; see also § 6.2.5.
In the Von Leeb case (The German High Command Trial) in 1948, the US Military Tribunal at Nuremberg stated: “Under general principles of law, an accused does not exculpate himself from a crime by showing that another committed a similar crime, either before or after the alleged commission of the crime by the accused.” 
United States, Military Tribunal at Nuremberg, Von Leeb case (The German High Command Trial), Judgment, 30 December 1947–28 October 1948.
In 1992, in its final report to Congress on the conduct of the Gulf War, the US Department of Defense stated:
Central Command … forces adhered to … fundamental law of war proscriptions in conducting military operations during Operation Desert Storm through discriminating target selection and careful matching of available forces and weapons systems to selected targets and Iraqi defenses, without regard to Iraqi violations of its law of war obligations toward the civilian population and 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.