Related Rule
United States of America
Practice Relating to Rule 47. Attacks against Persons Hors de Combat
Section A. General
The US Air Force Pamphlet (1976) states: “The law of armed conflicts clearly forbids the killing or wounding of an enemy who … is … hors de combat.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 4-2(d).
The Pamphlet goes on to say: “In addition to the grave breaches of the Geneva Conventions of 1949, the following acts are representative of situations involving individual criminal responsibility: … deliberate refusal of quarter.” 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 15-3(c)(3).
The US Naval Handbook (1995) provides: “The following acts are representative war crimes: … denial of quarter (i.e., killing or wounding an enemy hors de combat …).” 
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), § 6.2.5(4).
The US Naval Handbook (2007) states that examples of war crimes that could be considered as grave breaches of the 1949 Geneva Conventions include: “Denial of quarter (i.e., killing or wounding an enemy unable to fight due to sickness or wounds or one who is making a genuine offer of surrender).” 
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, § 6.2.6(4).
The US Manual for Military Commissions (2010), Part IV, Crimes and Elements, states: “The term ‘protected person’ means any person entitled to protection under one or more of the [1949] Geneva Conventions, including … military personnel placed out of combat by sickness, wounds, or detention.” 
United States, Manual for Military Commissions, published in implementation of Chapter 47A of Title 10, United States Code, as amended by the Military Commissions Act of 2009, 10 U.S.C, §§ 948a, et seq., 27 April 2010, § 1(a)(2)(B), p. IV-1.
The US Military Commissions Act (2006), passed by Congress following the Supreme Court’s decision in Hamdan v. Rumsfeld in 2006, amends Title 10 of the United States Code as follows:
§ 950v. Crimes triable by military commissions
“(a) DEFINITIONS AND CONSTRUCTION.—In this section:
“ …
“(2) PROTECTED PERSON.—The term “protected person” means any person entitled to protection under one or more of the Geneva Conventions, including—
“ …
“(B) military personnel placed hors de combat by sickness, wounds, or detention.
“ …
“ [4](B) death, damage, or injury incident to a lawful attack.
“(b) OFFENSES. The following offenses shall be triable by military commission under this chapter at any time without limitation:
“(1) Murder of protected persons.—Any person subject to this chapter who intentionally kills one or more protected persons shall be punished by death or such other punishment as a military commission under this chapter may direct. 
United States, Military Commissions Act, 2006, Public Law 109-366, Chapter 47A of Title 10 of the United States Code, 17 October 2006, p. 120 Stat. 2625, § 950v(a)(2)(B) and (4)(B).
The US Military Commissions Act (2009) amends Chapter 47A of Title 10 of the United States Code as follows:
§ 950p. Definitions; construction of certain offenses; common circumstances
“(a) DEFINITIONS.—In this subchapter:
“ …
“(2) The term ‘protected person’ means any person entitled to protection under one or more of the [1949] Geneva Conventions, including … military personnel placed out of combat by sickness, wounds, or detention. 
United States, Military Commissions Act, 2009, § 950p(a)(2).
In the Von Leeb case (The German High Command Trial) before the US Military Tribunal at Nuremberg in 1948, the accused, former high-ranking officers in the German army and navy, were charged, inter alia, with war crimes against enemy belligerents and prisoners of war in that they refused to give quarter to prisoners of war and members of armed forces of nations then at war with the Third Reich. The Tribunal stated:
When Allied airmen were forced to land in Germany, they were sometimes killed at once by the civilian population. The police were instructed not to interfere with these killings, and the Ministry of Justice was informed that no one should be prosecuted for taking part in them. 
United States, Military Tribunal at Nuremberg, Von Leeb case (The German High Command Trial), Judgment, 28 October 1948.