The relevance of IHL in the context of terrorism official statement by the ICRC 21 July 2005. "If civilians directly engage in hostilities, they are considered 'unlawful' or 'unprivileged' combatants or belligerents (the treaties of humanitarian law do not expressly contain these terms). They may be prosecuted under the domestic law of the detaining state for such action".
Article 51 (3) of Protocole I "Civilians shall enjoy the protection afforded by this section, unless and for such time as they take a direct part in hostilities". (Geneva Conventions Protocol I Article 51.3)