Let set aside duty and participation for a moment, let's assume the subject is engaging in an armed conflict.
The premier of the argument is the status of lawful combatant, to proof the subject is a lawful combatant means one need to proof the status the subject fighting for the belligerent country, that would mean the argument of whether or not AQ is the regonized fighting force for Taliban and to some extend, if AQ represent the country of Afghanistan.
If one can prove the co-relation of the above, one can establish that a Taliban fighter is a lawful combatant
The problem here is that the Taliban was never the legitimate authority figure in Afghanistan in the first place, at least not in the ways people commonly accepted as being characteristics of a 'legit' government.
A major component of governance is the ability to secure one's position from contestant authority figures and Afghanistan, all the way up to the time the US entered the country, was in a low intensity civil war from contestant authority figures scattered throughout the country. It is only when an authority figure established himself or itself as non-disputable can foreigners begins to find themselves bereft of alternatives for their petitions.
Korea cannot have two presidents, hence we have North and South Koreas. China cannot have two presidents, hence we have a civil war where one side was defeated and fled into physical exile away from the main geographical region that is coveted for being associated with what is China. Germany was divided post WW II into two politically distinct entities. For Afghanistan, the Taliban was occasionally defeated when it tried to assert its authority over the country. A famous anti-Taliban resistant leader was Ahmad Massoud. Up until 2001, even with the assistance of Pakistan, the Taliban could not pacify all of Afghanistan.
With no lawful or at least nominally recognized authority figure in Afghanistan, any combatant from inside Afghanistan that take up arms against a foreign state is an illegal combatant.
However, the case would be much much more complicated if you factor in the legitimacy of war and the nationality of the subject.
The war in Afghanistan was declared by US led NATO to the Taliban, the then de facto government of Afghanistan. The express objective would clear all AQ operative as lawful combatant INSIDE Afghanistan
The legitimacy of any war is independent of the status and characters of the participants. If a country have a legitimate grievance against another country, assuming victim status for now, even if the victim country hires mercenaries, who are always illegal combatants, the mercenary forces do not detract from the moral just cause of the war. Technically speaking, the WW II era famous Flying Tigers American airmen who fought in China for the Chinese people against Imperial Japan were -- mercenaries. China was righteously the victim then and American mercenaries took nothing away from China as the victim of Imperial Japan's aggression.
AQ was a guest organization under the generosity and protection of the Taliban inside territory controlled by the Taliban. AQ was never an official branch of service of any legitimate government in Afghanistan. As far as we know, AQ was never a mercenary force, which would be meaningless and render any AQ fighter an illegal combatant anyway. There is no credible interpretation of the accepted rules of war to render any AQ fighter a legal combatant.
However, that precedent ended when Taliban were expelled and replaced by the current government regime (Karzai Government). The belligerency can be arguably gone and NATO and US forces stationed in Afghanistan is of good will of the Karzai government. That would instantly relegate the AQ operator status into unlawful combatant, as the war enter the insurrection stage.
The premier now would change to whether or not the clause of war for the American has been gone, as the Afghanistan government has been changed hand, what the situation after have transformed into a Civil War, where US have officially lose the belligerency since the US war ended in Afghanistan
The problem here is that Afghanistan, as much as we can generously call it a 'country' in the political sense instead of the generic geographical context of the word 'country', was never a belligerent against the US despite 9/11, because in order to be regarded as a legal belligerent, a country must be unified under one authority figure in the first place, and history showed the Taliban was never that figure. The strongest armed force in Afghanistan, perhaps, but as long as challengers can petition foreigners for moral and even physical support and give the Taliban a proverbial 'good fight', the Taliban will never be that non-disputable authority figure.
That does not mean the US cannot be a belligerent against the Taliban, and by extension, against Afghanistan. After all, AQ being transnational entity and still needs a haven in order to recruit and train its fighters, and when the Taliban made portions of Afghanistan available for that purpose, Afghanistan became a legitimate target for US belligerency under...
The Avalon Project - Laws of War : Rights and Duties of Neutral Powers and Persons in Case of War on Land (Hague V); October 18, 1907
Laws of War :
Rights and Duties of Neutral Powers and Persons in Case of War on Land (Hague V); October 18, 1907
If AQ have a grievance against the US, the Taliban should not have injected themselves and Afghanistan into the mix. But by allowing AQ to use Afghanistan as a base from which ultimately 9/11 came to be, the Taliban stripped Afghanistan of any moral protection befitting a neutral state.
If I have to submit my own opinion, I would argue that AQ has no longer maintain their legal combatant status, and when you put in a US national fighting a war on the other side against the American to which American was no longer an official part of, this matter would instantly become an act of treason, which would relegate into Law Enforcement Affair.
Sorry...But I strongly disagree.
Never mind Afghanistan or Pakistan for now. Transnational law enforcement works only when there are sympathies among countries, as in those with a unified and strong singular authority figures who can speak for their peoples, for a set of laws and their associated crimes. The FBI and Interpol are useless in lawless countries like Sudan or Yemen where authority figures are monthly, if not weekly, fickle in their sympathies for the laws of their own lands, let alone laws agreed upon and shared by civilized societies.
It looks like you either did not know or forgot that our WTC Towers were twice victims of AQ: The 1993 underground garage truck bomb and finally Sept 11, 2001 where the towers finally came down.
AQ was not unknown to US before 9/11. AQ escalated the sophistication of attack in those 8 yrs. Prior to 9/11, we petitioned the Taliban in ways befitting one government to a peer for the Taliban to rid Afghanistan of AQ and Osama bin Laden. Then when 9/11 came, the US have every right to take the proverbial gloves off and treat AQ as a military force, albeit an illegal one that uses guerrilla warfare tactics.
But in the end, I am not a judge, this is merely my opinion
Do put yourself as a judge. By your opinions, you have and should have no fear doing it. You presented your opinions/arguments in a rational manner, much more than I can say for most of the uneducated and ignorant trolls here. Facts are meats and opinions are the seasonings that make the (intellectual) meals enjoyable.