This article  focuses on three key legal issues of particular relevance to the categorization of terrorism in a context of armed conflict, governed by international humanitarian law (IHL). Firstly, it examines IHL’s specific, narrow prohibitions on ‘terrorism’ in an armed conflict and the war crime of intending to spread terror amongst a civilian population, which is distinct from peacetime legal notions of terrorism. The war crimes jurisprudence has been developed by the International Criminal Tribunal for the former Yugoslavia (ICTY) and the Special Court for Sierra Leone, and has implications for criminal jurisdiction under customary IHL and before the International Criminal Court.

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Secondly, the paper analyses the varied and complicated relationships between IHL and international counter-terrorism law (CTL) norms and instruments. Depending on the norm and the context, CTL may apply, may not apply, or may apply only partially during armed conflict, and there is no general international rule determining whether CTL or IHL is the more special law (lex specialis) and should thus have priority. Often, CTL complements and extends IHL’s focus on preventing and criminalizing attacks on civilians. Furthermore, CTL often does not directly conflict with IHL. However, some aspects of CTL interfere with IHL’s delicate balance between humanitarian protection and military necessity, by ‘taking sides’, undermining the equality of the parties, and ultimately reducing incentives for non-state armed groups to comply with IHL.

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Finally, the paper concludes by exploring the adverse effects of CTL on humanitarian relief operations in armed conflicts. National implementation of CTL has variously chilled, restricted, prohibited and even criminalized humanitarian engagement by external actors with armed ‘terrorist’ groups. These measures have inhibited effective humanitarian assistance to vulnerable civilian populations, and undermined the confidence of non-state armed groups in humanitarian cooperation with the international community.

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The author argues that the generality and ambiguity in some IHL rules have enabled it to be flexibly, constructively and dynamically interpreted to accommodate emerging concerns, such as the threshold of a Non-International Armed Conflict (NIAC) or of Direct Participation in Hostilities (DPH). Furthermore, it is argued that where IHL has developed terrorism-specific rules, these have been focused on IHL’s core business of protecting civilians, rather than taking sides with states against non-state actors. This article’s relevance stems from its examination of developments in terrorism, counter-terrorism and IHL. It outlines the problems of criminalizing all fighting by armed groups in NIACs and shows that when CTL takes sides, it can undermine the already weak incentives for armed groups to respect IHL.