I’d like to turn our readers’ attention to the comment thread of Constantin’s post, which has raised a fascinating issue – when does an internal armed conflict become internationalized? I’d like to add a few thoughts of my own, first on some matters of definition.
We first need to agree on what the ‘internationalization’ of an internal armed conflict actually means. To my mind, that concept is only legally useful if it denotes the transformation of a prima facie non-international armed conflict into an international one, thereby rendering applicable to the said conflict the more comprehensive IAC legal regime. As is well known, there has been a long-standing trend – promoted, for example, by the case law of the ICTY and the ICRC customary law study – of arguing that most of IAC rules now apply to NIACs as well.
Crucially, however, at least one distinction between the two legal regimes remains. In IACs, the parties to the conflict are (at least) two equal sovereigns. Lawful participants in the hostilities who in effect represent those sovereigns thereby have combatant status, and enjoy the privilege of belligerency. They cannot be prosecuted by the other party for their mere participation in the hostilities, but solely for violations of IHL. In NIACs, however, the parties are fundamentally different – most commonly a government and a rebellious non-state actor. Because governments have every right to suppress rebellions against them, no combatant status or privilege exists in NIACs. A rebel can be prosecuted for the mere fact that he is a rebel, even if he has been completely observant of the rules of IHL. Thus, for example, the government of Afghanistan has every right to imprison a Taliban soldier, even if that soldier committed no war crime.
Note that this distinction is based on party structure to the conflict and is therefore here to stay. Note also that because the distinction between IACs and NIACs is based on party structure, one cannot logically first ask the question (as Federico does in the comments) whether there is an armed conflict simpliciter, and the ask further whether that conflict is international or non-international. Rather, IACs and NIACs are separate legal categories, neither of which is residual in nature, as it is impossible to establish either without making an inquiry into party structure. In other words, an ‘armed conflict’ exists when there is an IAC or a NIAC, not the other way around.
Per Common Article 2 of the Geneva Conventions, IACs are defined as conflicts between states. There are thus two basic ways of ‘internationalizing’ a NIAC: (1) for treaties and/or custom to exceptionally expand the definition of an IAC to include as parties some sufficiently state-like entities, or (2) for the non-state actor which is a party to a NIAC with a state to be considered as acting on behalf of a third state.