Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 65. Perfidy
Section A. General
The UK Military Manual (1958) states:
Good faith, as expressed in the observance of promises, is essential in war, for without it hostilities could not be terminated with any degree of safety short of the total destruction of one of the contending parties.
The borderline between legitimate ruses and forbidden treachery has varied at different times, and it is difficult to lay down hard and fast rules in the matter. Many of the doubtful cases, however, which arose at a time when, from the nature of their weapons, troops could only engage at close range, can now seldom or never occur. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 308 and 310.
The manual also notes, in connection with the requirements to be granted the status of combatant, that irregular troops “should have been warned against the employment of treachery”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 95.
The UK LOAC Pamphlet (1981) states that treachery “means tricking an enemy into believing that he is entitled to, or required to give, protection under international law, with intent to betray that confidence”. 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 4, p. 12, § 2(a); see also Annex A, p. 46, § 4.
The UK LOAC Manual (2004) states:
Perfidy is defined as “acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.9.1.
In its chapter on maritime warfare, the manual states:
Acts inviting the confidence of an adversary to lead it to believe that it is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence, constitute perfidy. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 13.83.
With regard to internal armed conflict, the manual states:
The definition of perfidy in paragraph 5.9.1 [relating to international armed conflicts, quoted above] may also be used as guidance as to the meaning of “treachery” in internal armed conflicts. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.12.1.
A training video on IHL produced by the UK Ministry of Defence describes as “complicated” the difference between ruses and treachery. 
United Kingdom, Ministry of Defence, Training Video: The Geneva Conventions, 1986, Report on UK Practice, 1997, Chapter 2.4.