Related Rule
United States of America
Practice Relating to Rule 47. Attacks against Persons Hors de Combat
Section C. Quarter under unusual circumstances of combat
The US Field Manual (1956) states:
A commander may not put his prisoners to death because their presence retards his movements or diminishes his power of resistance by necessitating a large guard, or by reason of their consuming supplies, or because it appears certain that they will regain their liberty through the impending success of their forces. It is likewise unlawful for a commander to kill prisoners on grounds of self-preservation, even in the case of airborne or commando operations. 
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, § 85.
In the Griffen case in 1968, a US Army Board of Review confirmed the sentence of unpremeditated murder for having executed a Vietnamese prisoner, following a “manifestly illegal” order to do so. The accused declared that “he felt that the security of the platoon would have been violated if the prisoner were kept, since their operations had already been observed by another suspect”. The Board of Review cited paragraph 85 of the US Field Manual prohibiting the killing of prisoners of war. It added that the “killing of a docile prisoner taken during military operations is not justifiable homicide”. 
United States, Army Board of Review, Griffen case, Judgment, 2 July 1968.
At the CDDH, the United States stated:
The word “feasible” when used in draft Protocol I, for example in Articles 50 and 51 [57 and 58], refers to that which is practicable or practically possible, taking into account all circumstances at the time, including those relevant to the success of military operations. 
United States, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.42, 27 May 1977, p. 241.