Nathalie Weizmann’s compendium of various views on the geographic scope of a non international armed conflict (NIAC) is extremely useful.
In addition to the listed papers, the ABA Center for Human Rights (in a discussion paper, which I drafted with Joe Onek) addressed a related but slightly different question concerning the geographic scope of the current US conflict with al-Qaeda: where would it be legal for the US to use lethal force against al-Qaeda or associated groups outside the recognizable war zone of Afghanistan and the border areas of Pakistan?
The paper concluded that international law does not permit the US, as part of the ongoing armed conflict with al-Qaeda, to carry out lethal operations against al-Qaeda or an associated armed group under the more permissive rules of international humanitarian law except in States where there is an ongoing armed conflict and al-Qaeda or an associated armed group is participating as a party to that conflict. This limitation on the geographic scope of the war against al-Qaeda protects against the expansion of armed conflict as well as against incursions on State sovereignty. (The paper also concluded that in those countries where there is no ongoing armed conflict involving al-Qaeda, a threat from al-Qaeda would have to be met with law enforcement or intelligence-gathering capabilities unless the threat were of such a magnitude as to constitute an “armed attack” against the US giving rise to the right to act in self-defense pursuant to Article 51 of the UN Charter independent of any right to use force as part of the ongoing armed conflict between the US and al-Qaeda.)
The paper called on the administration to resolve the ambiguity in its public statements on this issue and clearly state that it accepts this view of international law, which it appears (at least in large measure) to be following in practice. As readers know, that has not yet happened.