Let's start with the question of "sources"

Sources of power (and sources of restrictions on power; and sources of protections against the misuse of power) in the US - scarcely rocket science (I leave state laws out of this, for the moment, but they follow a similar pattern):

1. Federal Constitution; and

2. Federal "laws".

What are those "laws" ?

1. Statutes (e.g., UCMJ)

2. Regulations (e.g., DoD Directive 5525.7, implementing interagency co-operation agreement between DoD and DoJ re: Investigation and Prosecution of Certain Crimes)

3. Rules (e.g., Rules for Courts-Martial, R.C.M.)

4. Executive Orders (e.g., 1984 EO authorizing the Manual for Courts-Martial, as subsequently amended - see App 25 of 2008 MCM).

5. Treaties ratified by the Senate (e.g., 1949 GCs, but not 1977 APs); and Executive Agreements (2 kinds: Presidential, based solely on presidential powers; and Presidential-Congressional, based on joint powers).

6. "Common law" (domestic and international). Huge areas (mostly in civil actions) incorporate domestic common law (e.g., diversity cases where the rules of decision require Federal courts to refer to state substantive statues and common law decisions); and some international non-treaty law (e.g., admiralty, international insurance, commerce and financial).

In the criminal area, incorporation is generally via statute (e.g., Article 18 UCMJ incorporating international law, or Article 134 incorporating Title 18 statutes). A general Federal criminal common law has been rejected since the War of 1812. However, the "common" laws of war can enter the UCMJ via military commissions or tribunals as an adjunct to the UCMJ (and has been such since the Civil War) - e.g., Quirin (WWII executions of Uboat special ops) and Yama####a (post-WWII execution of general for war crimes committed by others).

So also, "customary international law" can be incorporated (by whom and by what process are issues); but, despite the love of I Law pundits for "customary international law", it has not been a favored source of law for SCOTUS (so far, at least).

So, the sources are the same for powers, restrictions on powers and protections against powers misused. In short, you have to look at the entire legal environment - sound familar ?

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Protections & Questions from Schmedlap

These are answered solely on the basis of existing US law, as presented by the Obama DoJ to the Federal courts; and the latter's acceptance or modification of that presentation.

Does an al-Qaeda operative derive his protections from the LOAC?
Yes; but let's look at what his protections are before becoming an al-Qaeda operative. He is then clearly a civilian (non-combatant) under the Hague-Geneva Conventions; he also could be a US citizen, legal resident alien, unlawful resident alien or a non-resident (different protections depending on his status). Let's take the simple non-resident case (Akmed in Astan). While a civilian, he is entitled to e.g., the protections of GC IV.

If so, can he surrender these rights by disregarding the LOAC?
Yes, but only to the extent that he ceases to be a civilian. So, Ahmed becomes an "AQ operative" - what does that mean ? If he picks up an AK, no question that he is a combatant (not a "regular combatant", but an "irregular combatant" because AQ flunks Common Article 2 by not accepting or applying the GCs).

That means that under the 2001 AUMF and LOAC (if the ROE/RUFs are not more restrictive - which is a big if, about which everyone is aware), he can be killed or captured at any time and place. That kill or capture rule applies even if he is not committing a hostile act or is not exhibiting hostile intent by an overt threat. In short, he can be popped even if he has put down the AK and is taking a leisure moment at the latrine.

Now, kill or capture at any time or place is the same default rule (absent more restrictive ROE/RUFs) that we would have in a declared war against "regular combatants" (e.g., the Wehrmacht in WWII). There is a difference in the capture part of the equation.

From a legal standpoint (under existing statutes and treaties accepted and applied by the US), every person who is detained in an area of armed conflict must fall into one of three general classifications (there are sub-categories):

1. Persons who meet the criteria of EPWs and some other defined categories (requiring reference to and meeting the requirements of Common Article 2, and then going on to the requirements of Article 4 et seq of GC III).

2. Persons who meet the criteria of civilians and some other defined categories (requiring reference to and meeting the requirements of Common Article 2, and then going on to the requirements of Article 4 et seq of GC IV).

3. Persons not meeting the requirements of either #1 or #2 and who fall within the scope of Common Article 3 (the armed conflict not of an interenational character "mini-convention").

See this post for more.

But, Ahmed doesn't like AKs. However, he is more than willing to cook for the AQ fighters; transport them, their ammo, weapons and other supplies; and carry out ISR for them. Or, he happens to have some education and is one very good explosive production man (just makes them; total klutz at placing them). Or, having lived long in the "West" (or being an expat US citizen) becomes an expert in agitprop and using infofare vs the US. Or, having left the AK (well, not quite since it's still on the wall), he becomes a planner of operations - or, on an even higher level, a strategist and policy maker. Or, not being good at any of the above, becomes a financier of AQ.

Now, my readers, where do you want to draw the line between "irregular combatants" (unprotected from kills; but still having Common Article 3 protections if captured) and "civilians" (protected from kills and having full GC IV protections), in all of these variant Ahmed situations ? You tell me.

Are our legal powers derived from the LOAC? Or are our powers derived from our Constitution and those powers limited on the international stage by treaties that we are a party to?
Answered above as to sources (of powers and limitations).

If an operative surrenders his protections, does this mean that our powers are expanded and we may handle that individual in a manner inconsistent with the LOAC? Let's say that the answer is no.
If an "operative" loses his "civilian" status and becomes an "irregular combatant", our powers obviously expand in the kill or capture (any time, any place - if more restrictive ROE/RUFs are not in effect); and some tactics (e.g., pseudo ops) may be legal (wisdom is another issue). No individual, organization or war (armed conflict) may be handled in a manner inconsistent with the LOAC.

For the sake of argument, assert that the LOAC does not govern our actions on an individual-by-individual or organization-by-organization basis. It governs our actions in war, period. ....
The premise asserted just muddies the waters - LOAC create protections and put restraints on the actors. Repeat, No individual, organization or war (armed conflict) may be handled in a manner inconsistent with the LOAC.

My hunch is that it is both. In other words, it is sometimes redundant. If individual Y surrenders his protections, he nonetheless enjoys the benefits of those protections because of the redundancy of the LOAC, which continues to restrain Z actor. This seems problematic to me.
Not problematic at all. By changing his status, Y is covered by some rules which are different (cuz status changed) and some which are the same (cuz they cover regardless of status). Same for Z.

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Domestic and KSM

Suppose in the US there is a criminal law against shooting people and a civil law giving recourse to someone who gets shot. ....
The "guy" does not "waive his right to protection from the criminal law". The guy, by shooting up the mall, has changed his status from that of a non-criminal to that of a criminal - and becomes subject to the criminal law, although he still retains most of his rights as a US citizen or US resident (he loses some if convicted).

Yes, he can sue the cop that shoots him (or his family can if he's killed). That takes only a lawyer who is willing to take the case and a filing fee. If properly defended (insurance companies and municipalities have settled far too much), that case has a dismal future. Most perps have a bad record - and many things to hide. These are civil actions. So, discover the hell out of the defendant and all of his witnesses. After they have taken the 5th a few dozen or 100s of times, the case should be in shambles.

And, usually the cop (probably cops) involved have sustained physical and/or mental injuries (hire Grossman as an expert witness). So, counterclaim and continue nasty discovery regardless of what the insurer or municipality does. Cops who aggressively counter-attack will generally do better before most juries than perps.

That seems to be the situation with the LOAC. Khalid Sheikh Mohammed is a known turd ....
Same idea - change of status and different rules. You have to look at the complete legal environment - which rules apply depends on the specific context.

KSM and his friends have stated that they wish to surrender all protections and accept execution. Normally I don't suggest going with terrs' wishes, but this must be kind to man-eating tigers' week. I think that is a fine idea on KSM's part. I wonder if he would go through with it ?