Related Rule
United States of America
Practice Relating to Rule 154. Obedience to Superior Orders
The US Field Manual (1956) states: “Members of the armed forces are bound to obey only lawful orders.” 
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, § 509(b).
The US Air Force Pamphlet (1976) states: “Members of the armed forces are bound to obey only lawful orders.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-4(d).
The US Soldier’s Manual (1984) tells the soldier: “Although you are responsible for promptly obeying all legal orders issued by your leader, you are obligated to disobey an order to commit a crime.” 
United States, Your Conduct in Combat under the Law of War, Publication No. FM 27-2, Headquarters Department of the Army, Washington, November 1984, p. 26.
The US Naval Handbook (1995) provides:
Members of the naval service, like military members of all nations, must obey readily and strictly all lawful orders issued by a superior. Under both international law and U.S. law, an order to commit an obviously criminal act, such as the wanton killing of a noncombatant or the torture of a prisoner, is an unlawful order and will not relieve a subordinate of his responsibility to comply with the law of armed conflict. 
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), § 6.1.4.
[emphasis in original]
The US Naval Handbook (2007) states:
Members of the naval service, like military members of all nations, must obey readily and strictly all lawful orders issued by a superior. Under both international law and U.S. law, an order to commit an obviously criminal act, such as the wanton killing or torture of a prisoner, is an unlawful order and will not relieve a subordinate of his responsibility to comply with the law of armed conflict. 
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, § 6.1.4.
[emphasis in original]
In the Calley case in 1973, the US Army Court of Military Appeals approved the following instructions given to the panel by the trial judge in a case where the accused invoked an order to kill unresisting detainees:
A determination that an order is illegal does not, of itself, assign criminal responsibility to the person following the order for acts done in compliance with it. Soldiers are taught to follow orders, and special attention is given to obedience of orders on the battlefield. Military effectiveness depends upon obedience to orders. On the other hand, the obedience of a soldier is not the obedience of an automaton. A soldier is a reasoning agent, obliged to respond, not as a machine, but as a person. The law takes these factors into account in assessing criminal responsibility for acts done in compliance with illegal orders. 
United States, Army Court of Military Appeals, Calley case, Judgment, 21 December 1973.
The Court cited a writer’s opinion to the effect that:
For the inferior to assume to determine the question of the lawfulness of an order given him by a superior would of itself, as a general rule, amount to insubordination, and such an assumption carried into practice would subvert military discipline. Where the order is apparently regular and lawful on its face, he is not to go behind it to satisfy himself that his superior has proceeded with authority, but is to obey it according to its terms, the only exceptions recognized to the rule of obedience being cases of orders so manifestly beyond the legal power or discretion of the commander as to admit of no rational doubt of their unlawfulness
Except in such instances of palpable illegality, which must be of rare occurrence, the inferior should presume that the order was lawful and authorized and obey it accordingly, and in obeying it can scarcely fail to be held justified by a military court. 
United States, Army Court of Military Appeals, Calley case, Judgment, 21 December 1973, referring to Col. William Winthrop, Military Law and Precedents, pp. 296–297, 2nd edition 1920 (reprint).
[emphasis in original]
In 1995, in the Huet-Vaughn case before the Court of Appeals for the Armed Forces, the US Government successfully appealed the decision of the Court of Military Review that had set aside the findings and the sentence imposed by a court martial that had found Huet-Vaughn (a medical officer in the US Army Reserve) guilty of desertion with intent to avoid hazardous duty and shirk important service. The Court of Appeals stated:
To the extent that CPT Huet-Vaughn’s acts were a refusal to obey an order that she perceived to be unlawful, the proffered evidence was irrelevant. The so-called “Nuremberg defense” applies only to individual acts committed in wartime; it does not apply to the Government’s decision to wage war. See United States v. Berrigan, 283 F. Supp. 336, 341 (D.Md. 1969), aff’d sub nom. United States v. Eberhardt, 417 F.2d 1009 (4th Cir. 1969), cert. denied, 397 U.S. 909 (1970). The duty to disobey an unlawful order applies only to “a positive act that constitutes a crime” that is “so manifestly beyond the legal power or discretion of the commander as to admit of no rational doubt of their unlawfulness.” United States v. Calley, 22 U.S.C.M.A. 534, 543, 48 C.M.R. 19, 28 (1973), citing Lambert v. California, 355 U.S. 225, 228, 2 L. Ed. 2d 228, 78 S. Ct. 240 (1957), and W. Winthrop, Military Law and Precedents 296–97 (2d ed. 1920 Reprint). CPT Huet-Vaughn tendered no evidence that she was individually ordered to commit a “positive act” that would be a war crime. 
United States, US Court of Appeals for the Armed Forces, Huet-Vaughn case, Judgment, 28 September 1995.
According to the Report on US Practice, it is the opinio juris of the United States that the law of war obliges all persons not to commit war crimes. The duty to obey the law of war prevails over the duty to obey a manifestly unlawful order. 
Report on US Practice, 1997, Chapter 6.8.