United States of America
Practice Relating to Rule 65. Perfidy
Section I. Simulation of civilian status
According to the US Field Manual (1976), the use of civilian clothing by troops to conceal their military character during battle is an act for which a combatant would lose his right to be treated as a prisoner of war. 
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, § 74.

The manual also states: “In addition to the ‘grave breaches’ of the Geneva Conventions of 1949, the following acts are representative of violations of the law of war (‘war crimes’): … use of civilian clothing by troops to conceal their military character during battle.” 
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, § 504(g).

According to the US Air Force Pamphlet (1976), the use of civilian clothing by troops to conceal their military character during battle is an act for which a combatant would lose his right to be treated as a prisoner of war. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 7-2.

The Pamphlet further states: “In addition to the grave breaches of the Geneva Conventions of 1949, the following acts are representative of situations involving individual criminal responsibility: … intentional use of civilian clothing to conceal military identity during battle.” 
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)(6).

In respect of air warfare, the Pamphlet states:
Aircrew members do customarily wear uniforms because flight suits fully qualify as uniforms when they are so distinctive in character as to distinguish the wearer from the civilian population … In that connection, the prohibition of perfidy, such as disguising oneself as a civilian in order to engage hostilities, … is applicable. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, § 7-3(a).

It also provides that, generally speaking, “disguising combatants in civilian clothing in order to commit hostilities constitutes perfidy”. This is also the case of the “feigning by combatants of civilian, noncombatant status”. 
United States, Air Force Pamphlet 110-31, International Law – The Conduct of Armed Conflict and Air Operations, US Department of the Air Force, 1976, §§ 8-6(a) and 8-3(a).

The US Instructor’s Guide (1985) states: “In addition to the grave breaches of the Geneva Conventions, the following acts are further examples of war crimes: … using civilian clothing to conceal military identity during battle.” 
United States, Instructor’s Guide – The Law of War, Headquarters Department of the Army, Washington, April 1985, p. 13.

The US Naval Handbook (1995) states that illegal combatants may be denied prisoner-of-war status, tried and punished. It also states: “It is a violation of the law of armed conflict to kill, injure, or capture the enemy by false indication of … civilian status … Attacking enemy forces while posing as a civilian puts all civilians at hazard. Such acts of perfidy are punishable as war crimes.” 
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), §§ 12.7. and 12.7.1.

In 1942, in the Quirin case in which German saboteurs had entered the United States in civilian clothing, the US Supreme Court held:
Each petitioner, in circumstances which gave him the status of an enemy belligerent, passed our military and naval lines, in civilian dress and with hostile purpose. The offense [under the laws of war] was complete when with that purpose they entered – or, having so entered, they remained upon – our territory in time of war without uniform or other appropriate means of identification. 
United States, Supreme Court, Quirin case, Judgment, 31 July 1942, and Extended Opinion, 29 October 1942.

US practice since the Second World War has refused prisoner-of-war treatment to enemy combatants captured in civilian clothing while not carrying their arms openly. During the Vietnam War, the US policy was to consider that all combatants captured during military operations were to be accorded prisoner-of-war status, while terrorists, spies and saboteurs were not. 
George S. Prugh, Law at War: Vietnam 1964-1973, Department of the Army, Vietnam Studies, Washington D.C., 1975, p. 66.

In 1989, in a memorandum of law, the Judge Advocate General of the US Department of the Army stated:
Traditionally, soldiers have an obligation to wear uniforms to distinguish themselves from the civilian population. Law-of-war sources prior to World War II suggested that the prohibition on killing or wounding “treacherously” referred to soldiers disguising themselves as civilians in order to approach an enemy force and carry out a surprise attack. That concept was thrown into disarray during World War II by the reliance on partisans by all parties to that conflict. While frequently characterized as an assassination, the 27 May 1942 ambush of SS General Reinhard Heydrich by British SOE [Special Operations Executive]-trained Czechoslovakian partisans is representative of the practice of each party to the conflict employing organized resistance units to carry out attacks against military units and personnel of an occupying power.
Reliance upon organized partisan forces changed state practice and, accordingly, the law of war. Coordinated British and U.S. revisions of their respective post-World War II law of war manuals reflected this change. For example, the following … italicized … sentence was added to paragraph 31 [of the US Field Manual]:
[Article 23(b) of the 1907 Hague Regulations] is construed as prohibiting assassination … It does not, however, preclude attacks on individual soldiers or officers of the enemy whether in the zone of hostilities, occupied territory, or elsewhere.
The annotations to [the manual] state that the [italicized] sentence was inserted “so as not to foreclose activity by resistance movements, paratroops, and other belligerents who may attack individual persons”. The deliberate decision by many nations to employ surrogate guerrilla forces in lieu of or in connection with conventional military units to fight a succession of guerrilla wars since 1945 has served to raise further doubts regarding the traditional rule.
While state practice suggests that the employment of partisans is lawful, that is, would not constitute assassination, a question remains regarding the donning of civilian clothing by conventional forces personnel for the purpose of killing enemy combatants. However, in the one known case of such practice during World War II, a British officer who successfully entered a German headquarters dressed in civilian attire and killed the commanding general was decorated rather than punished for his efforts. 
United States, Department of the Army, Office of the Judge Advocate General, Memorandum of Law: Executive Order 12333 and Assassination, 2 November 1989, The Army Lawyer, Pamphlet 27-50-204, December 1989, p. 6.
[emphasis in original]
According to the Report on US Practice, the opinio juris of the United States is that:
Customary international law does not … prohibit belligerents from using saboteurs, secret agents or other irregular forces feigning civilian status to attack legitimate military targets. Wear of civilian clothing during an attack, or during a spying or sabotage mission behind enemy lines, may subject combatants to punishment if captured by the enemy. 
Report on US Practice, 1997, Chapter 2.4.