Here is a starting point (for what ended up a longer screed than I intended):

from John
By current "laws" all insurgents who don't fight openly - 99% - are illegal combatants and most (90%+?) are terrorists at one time or another - making them simply criminals.
As I view this statement (with which I have much agreement in end result), it mixes two different things:

(1) How to treat combatants under the Laws of War (kill, wound or detain); and

(2) How to treat the same people under the Rule of Law (criminal prosecutions).

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As to the first thing, there are two different streams of law:

(1) 1949 GCs, where in US law these insurgent combatants are not protected combatants (GC III does not apply, but Common Article 3 does), and are certainly not protected civilians (GC IV does not apply). One of them can claim either GC III or GC IV status; and if so is entitled to an fair hearing on that claim. Otherwise, that combatant is entitled only to the rights under Common Article 3 - which are not the best-defined in the world.

(2) 1949 GCs + the 1977 Protocals I and II. While those Protocals may be subject to interpretation, one reading is that an insurgent combatant can turn on and off his or her combatant or civilian status - but, regardless of which status, is still protected either as a combatant or civilian. In that view, the ANC folks, being part of a "national liberation struggle", were lawful combatants or lawful civilians - depending on which costume they decided to wear at the time. Most NATO nations accept the Protocals. Interestingly enough, the nations where we now have armed conflicts (Iraq through India) do not accept the Protocals.

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As to the second thing, I see use of "terrorism" to define a crime as problematic - simply because attempts at drafting definitions of "terrorism" have been notably unsuccessful. In terms of US constitutional law, those definitions tend to flunk either the void for vagueness test, the overbreadth test or both.

Still, like pornography, we do have a grasp of what terrorism is - we know it when we see it, even if we can't define it. Moreover, the acts of combatants (if they are unprotected under the 1949 GCs) can certainly be criminal acts, as well as acts of armed conflict.

My suggested path out of this morass is basically KISS - relying on US constitutional law as the fundamental guide (I'm sworn to support that, not the views of the Eminent Jurists).

As to the first point (the Laws of War), the power to declare and conduct armed conflicts[*] is vested in our Executive and Legislative branches (leaving aside the relative powers of each in that area, the overall power cannot be questioned). There is no limitation that that power can only be exercised against State actors who commit acts of armed conflict against us. It is up to the Executive and Legislative branches to decide what in any particular case constitutes acts of armed conflict within that power, and what State or non-State actor or actors are responsible for those acts of armed conflict. Those acts may or may not constitute "terrorism" - the definition of which is not material to the existence of an armed conflict (which is determined in the discretion of our constitutionally designated bodies - hopefully in their sound discretion, but determined none the less).

The criminal law issues then fall into place, once we determine under our (US) law whether the combatants on the other side are lawful or not under the 1949 GCs (see, e.g., FM 27-10) that we have ratified. If they have lawful status, their combatant acts in the armed conflict are protected from criminal prosecution (lawful combatant immunity); and, if detained, have PW/POW status. If they do not have lawful status, their combatant acts in the armed conflict are not protected and they may be prosecuted criminally; and, if detained, would be subject to Common Article 3 status [**].

The criminal law charges are vanilla - murder, assault, attempts to do the same and conspiracy are all well-defined crimes in our domestic law. One could (as in the case of KSM) add some of the Nuremberg charges. E.g., I view 9/11 as a crime against humanity - as well as a planned attack by combatants (unlawful) against strategic targets in an armed conflict. Whether those crimes should be tried in Federal court, or in special courts under different procedural and evidentiary rules, is a separate question as to which reasonable people can differ as to the pros and cons.

As a final note, a criminal statute could be enacted making it a criminal act for individual combatants to engage in an armed conflict with the US, where they are not protected by combatant immunity under the GCs as we have ratified and interpreted them. So far, we have not done that; but have provided that they may be detained for the duration of the armed conflict in which they were involved.

In conclusion, my view is that "terrorism" is not a good term - either to define legal standards, or for that matter to define the nature of an armed conflict.


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[*] Yes, the literal phrase is "declare war", but from the gitgo that was read by US jurists (e.g., Kent's Commentaries from the early 1800s) not to require a formal declaration of war:

LECTURE III. OF THE DECLARATION, AND OTHER ABLY MEASURES OF A STATE OF WAR.
....
2. Declaration of War.
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But though a solemn declaration, or previous notice to the enemy, be now laid aside, it is essential that some formal public act, proceeding directly from the competent source, should announce to the people at home their new relations and duties growing out of a state of war, and which should equally apprise neutral nations of the fact, to enable them to conform their conduct to the rights belonging to the new state of things. War, says Vattel, [(b)] is at present published and declared by manifestoes. Such an official act operates from its date to legalize all hostile acts, in like manner as a treaty of peace operates from its date to annul them. As war cannot lawfully be commenced on the part of the United States without an act of Congress, such an act is, of course, a formal official notice to all the world, and equivalent to the most solemn declaration.
Thus, there must be an AUMF act by Congress, which may be short of a formal declaration of war - as Kent points out by examples from the times before and after the Constitution was adopted (footnotes omitted below - see full text at link above for context):

Since the time of Bynkershoek, it has become settled by the practice of Europe that war may lawfully exist by a declaration which is unilateral only, or without a declaration on either side. It may begin with mutual hostilities. [(a)] After the peace of Versailles, in 1763, formal declarations of war of any kind seem to have been discontinued, and all the necessary and legitimate consequences of war flow at once from a state of public hostilities, duly recognized and explicitly announced by a domestic manifesto or state paper.

In the war between England and France, in 1T78, the first public act on the part of the English government was recalling its minister; and that single act was considered by France as a breach of the peace between the two countries. There was no other declaration of war, though each government afterwards published a manifesto in vindication of its claims and conduct. The same thing may be said of the war which broke out in 1793, and again in 1803; and, indeed, in the war of 1756, though a solemn and formal declaration of war, in the ancient style, was made in June, 1756, vigorous hostilities had been carried on between England and France for a year preceding.

In the war declared by the United States against England, in 1812, hostilities were immediately commenced on our part [{55}] as soon as the act of Congress was passed, without waiting to communicate to the English government any notice of our intentions. [(x)]
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[**] The Executive and Legislative branches have overall constitutional power (again leaving aside the issues of the powers held by each branch) to abrogate, amend or clarify the GCs. That may be unwise, and a breach of I Law, in a particular case; but, they (acting jointly) clearly have that power. It is difficult for many people around the World (e.g., the Eminent Jurists) to grasp that the Constitution, not I Law, is the Supreme Law of our land.