The LOAC is triggered by the existence of an armed conflict. If there is no armed conflict, the LOAC does not apply and domestic law will govern.
There are two types of armed conflicts, International Armed Conflicts and Non-international Armed Conflicts. No gap exists between them; either one or the other exists in the event of armed hostilities. There is no such thing as unregulated hostilities.
- International Armed Conflicts (IAC) are conflicts between States. The linchpin to determining if an IAC exists is to ask “is there a conflict between states that has lead to the intervention of armed forces?” The scope and duration of the conflict does not matter; nor does it matter if there has been a formal declaration of war. If there is a conflict between States that has lead to the intervention of armed forces, an IAC exists. IACs fall under the Geneva Conventions through Common Article 2. According to Common Article 2, all four (4) Geneva Conventions, as well as both of the Two Additional Protocols, apply in their entirety to an IAC.
- Non-international Armed Conflicts (NIAC) are all other types of armed conflicts. The law governing NIACs was originally written in response to horrific civil wars. NIACs fall under the Geneva Conventions through Common Article 3 which requires that all people be treated humanely, and the wounded and sick are cared for. Now, under customary international law, all of the main LOAC principles apply in a NIAC. However, the requirement to treat a captured enemy as a Prisoner of War, and the protection afforded to an enemy belligerent not be tried as a criminal for activities carried out during the armed conflict (privilege of belligerency), do not apply in a NIAC.
The Law of Armed Conflict (LOAC) 
At what point in your opinion does LOAC apply to the current armed insurrection in the US, considering we have officially be at war since 9/11? At what point does an attempted coup rise to the level required? Would the domestic enemy combatants need to specifically hold allegiance to the group that war was declared against? or does allegiance to like principles count Like “death to America”?
Gary,
Interesting question. I will do my best to answer it, but will have to do so “generally,” as I am not sure what armed insurrection (in the U.S.) you are referring to. In any event, here we go …
(1) We have been at “war” (in an armed conflict) since 9/11/2001. However, the authorization for that armed conflict basically comes from the “AUMF” (at least, it comes from the AUMF in a general sense, without getting into the executive’s powers, etc.). However, the AUMF authorizes force against those who had something to do with the 9/11 attacks, so it would have very little to do with an armed insurrection within the U.S.
AUMF (Sept. 18, 2001): https://www.congress.gov/107/plaws/publ40/PLAW-107publ40.pdf (and I believe this has been amended since 2001).
(2) Having said that, there is a difference between an international armed conflict (IAC) and a non-international armed conflict (NIAC). An IAC is what most think of when they hear the word “war,” and an IAC triggers all aspects of the LOAC. Conversely, a NIAC (something like an armed insurrection or Civil War) does not necessarily trigger all aspects of the LOAC.
In fact, the first question we must ask is whether the “armed insurrection” rises to the level of an “armed conflict” or “war / civil war,” etc. This is a de facto standard, meaning an insurrection becomes an armed conflict when the two sides are treating it like one. At this point in time, I don’t see any actions in the U.S. that would rise to the level of an armed conflict (as a NIAC) and thus the LOAC would not be implicated in any situation currently going on within the U.S. (unless, of course, you can tie the actions back to the 9/11 attacks … but even then, that is an entirely different and complicated matter, as the U.S. has had some 9/11 attackers come to the U.S. and we have handled them in a way that I would consider different than what the LOAC allows).
(3) You also asked: “Would the domestic enemy combatants need to specifically hold allegiance to the group that war was declared against?”
A couple of things need to be said here in order to answer the question. First, members of a non-state party (like rebels in a NIAC) are not “enemy combatants.” This is because they have no authority to take-up arms against the State, and thus they are illegal participants, and are not entitled to “combatant” status. This does not mean that their cause is not “just;” and it only means that they are fighting without any legal protection. For this reason, I would refer to them as “unlawful belligerents.”
As for a declared state of war … we don’t have that now, nor do we need a declared state of war in order for the LOAC to apply. All you need is for an armed conflict to take place; whether it is properly authorized or not, is a different question. However, once an armed conflict arises, regardless of how it came about, the LOAC can be implicated.
Having said that, you asked if the domestic fighters need to hold allegiance to the group that the U.S. is fighting against (?). If I understand your question correctly, the answer is “yes.” In other words, let’s say there is an actual armed insurrection within the U.S., and the U.S. enters into an armed conflict with this group (and thus we have a NIAC; and certain parts of the LOAC are triggered). The U.S. can only engage with those people fighting in the NIAC as part of the insurrection … however, that is a blurry line, I know … as it can be hard to tell who is part of the insurrection. It is for this reason that those fighting on behalf of the insurrection movement are not entitled to combatant status and have no privilege or legal standing to engage in combat.
There is another principle under the LOAC called “Direct Participation in Hostilities” (or DPH). To describe this generally, you have civilians who are not part of combat, and thus cannot be targeted. However, if a civilian were to take DPH (i.e. pick up a gun and start shooting), they would be targetable for so long as they were taking DPH. As soon as they stop, they are not longer a target. This is a necessary aspect of the LOAC but it can be problematic for those trying to “follow the rules.”
However, the point is that in an NIAC (or insurrection) there is likely to NOT be any state of war declared against anyone. What is most likely is that a particular group will decide to take-up arms against the State, and the State will respond … usually the State will respond with a Police style of force and put down the insurrection as merely criminal behavior. However, in the event the State responds with military force, you will have an armed conflict (NIAC in character). Further, the people participating in the insurrection would be unlawful belligerents, and would not likely have an organized group to which they belonged … but so long as they were distinguishable from the general civilian population, that is all that would matter (and it would matter not who they swore allegiance to). However, in the off-chance you could not tell if a person was part of the insurrection or not, it would be best to err on the side of caution and assume they were a civilian … and if they are caught taking DPH, then their status would change for so long as they were taking DPH.
(4) You also asked: “Is allegiance to a principle like ‘Death to America’ sufficient?”
This is a great question and I think I answered it above in number (3). I say that I think I answered it b/c in a NIAC, you are not normally going to have an organized group that is causing the insurrection. When I say “organized,” I really mean a cohesive group from top-to-bottom; as you might have many organized groups participating together, due to the fact that they share a common principle, etc. However, it may not be one large group. Does that make sense?
In any event, if the two sides of a conflict get to a point that they find themselves in a full-on armed conflict (even if just a NIAC), then the laws of the LOAC are triggered, regardless of how the insurrection group is defined or organized.
(Off the top of my head, the only time the organization of a group in an armed conflict would really matter would be if you were analyzing an armed conflict as an IAC … as the organization would play into the principle of distinction and combatant privilege.)
Conclusion:
So, that was a long answer and I am really just shooting from the hip here … however, as you can see, the analysis required to properly answer your question would require a lot more facts as to what the situation was, who was involved, etc.