The following conditions shall not be considered as depriving a medical unit or establishment of the protection guaranteed by Article 19:

(1) That the personnel of the unit or establishment are armed, and that they use the arms in their own defence, or in that of the wounded and sick in their charge.

(2) That in the absence of armed orderlies, the unit or establishment is protected by a picket or by sentries or by an escort.

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:

(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.

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.

The medics in the rescue force were not feigning their Red Cross status, the evacuation helicopter was not feigning its right to show the Red Cross. The other ruses are not prohibited.

The rough disguise of the helicopter as a civilian aircraft does not violate section 1(a) because the objective of the ruse was clearly not to gain the "protection of international law" (ie, keep the terrorists from shooting at it) but to mislead the terrorists into believing that it was part of a friendly force; that is, a lawful ruse de guerre.