In the context of war, perfidy is a form of deception, in which one side promises to act in good faith (e.g., by raising a flag of surrender) with the intention of breaking that promise once the enemy has exposed themselves (e.g., by coming out of cover in order to capture the surrendering forces).
The practice is specifically prohibited under the 1977 Protocol I Additional to the Geneva Conventions of 12 August 1949, which states:
Article 37.-Prohibition of perfidy
1. It is prohibited to kill, injure or capture an adversary by resort to perfidy. 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, shall constitute perfidy. The following acts are examples of perfidy:2. Ruses of war are not prohibited. Such ruses are acts which are intended to mislead an adversary or to induce him to act recklessly but which infringe no rule of international law applicable in armed conflict and which are not perfidious because they do not invite the confidence of an adversary with respect to protection under that law. The following are examples of such ruses: the use of camouflage, decoys, mock operations and misinformation.
(a) The feigning of an intent to negotiate under a flag of truce or of a surrender;
(b) The feigning of an incapacitation by wounds or sickness;
(c) The feigning of civilian, non-combatant status; and
(d) The feigning of protected status by the use of signs, emblems or uniforms of the United Nations or of neutral or other States not Parties to the conflict.
Perfidy was part of the customary laws of war long before the prohibition of perfidy was included in Protocol I. For example in Hague IV: Laws and Customs of War on Land (October 18, 1907), Article 23 includes:
In addition to the prohibitions provided by special Conventions, it is especially forbidden - ... To kill or wound treacherously individuals belonging to the hostile nation or army; ... To make improper use of a flag of truce, of the national flag or of the military insignia and uniform of the enemy, as well as the distinctive badges of the Geneva Convention; ...
The issue of whether the donning of enemy uniforms in order to approach the enemy without drawing fire was within the laws of war was established under international humanitarian law at the trial in 1947 of the planner and commander of Operation Greif, Otto Skorzeny, at the Dachau Trials. The court did not find Skorzeny guilty of a crime by ordering his men into action in American uniforms. He had passed on to his men the warning of German legal experts, that if they fought in American uniforms, they would be breaking the laws of war, but they probably were not doing so just by wearing the uniform. During the trial, a number of arguments were advanced to substantiate this position and that the German and US military seem to be in agreement on it. In its judgement the Court noted that the case did not require that the Court make findings other than those of guilty or not guilty, so consequently no safe conclusion could be drawn from the acquittal of all accused.
See also 
- Source: Law Reports of Trials of War Criminals. United Nations War Crimes Commission. Vol. IX, 1949: Trial of Otto Skorzeny and others General Military Government Court of the U.S. zone of Germany 18 August to 9 September, 1947