In my opinion, if you engage in war against the United States, simply possessing U.S. Citizenship does not provide a special protection from warfare, nor does it restrict the U.S. government from engaging in warfare.
I think you have provided the perfect example.
We are not talking about collateral damage (or deaths) here but the targeting of an American citizen for assassination. A better example would have been FDR targeting Tokyo Rose for assassination for traitorous acts during war. I don’t think any one back then would have batted an eye at something like that.
I would say that Presidents have always had the power to have people killed on foreign and domestic soil (en mass in some cases) so I won’t say that this is any different. If you dance with the elephants expect to get stepped on if you’re a pigmy.
Ergo the importance of congress declaring war (and not just consenting to presidential “police actions”).
If war has not been declared, no justification for whacking someone as an “enemy combatant” because there aren’t any.
This is certainly true. However when we say "warfare" we mean a declared and recognized war, and we understand that warring sides have chosen to wage that war and they are sufficiently aware of their actions. Every soldier has an option to not fight (he might be killed for that, but he won't become a killer himself.) The state's legislators are supposed to give the military the power to repel the invasion (or to invade) until specific military goals are achieved, while obeying traditional and mutually agreed rules of warfare. For example, killing of noncombatants is not allowed; torture is not allowed; killing of captured enemy soldiers is not allowed; and so on.
The problem here is that the decision to assassinate a person was made in private, and the target was not given a chance to surrender. Perhaps he would, perhaps he wouldn't - but we will never know that. This is the reason why when someone is wanted and is known to be in a house the police used to come to the house and offer terms of surrender.
In other words, a warfighter is a legitimate target. However the said warfighter must be recognized as such. You can't send an army into the nearest Wal-Mart, shoot everyone inside and then say that they were all terrorists. You can't even throw a grenade into a room where known but unarmed thieves are planning a robbery.
The reason for that is the same reason that led to creation of Magna Carta:
"The 1215 charter required King John of England to proclaim certain liberties, and accept that his will was not arbitrary, for example by explicitly accepting that no "freeman" (in the sense of non-serf) could be punished except through the law of the land, a right which is still in existence today."
Any ruler who is the sole arbiter of life and death of his people is known today as a dictator. It does not matter if he discusses his hit list with any of his hired henchmen because the result is entirely predictable. Only the neutral, fairly selected lawmen - judges and juries - should have a say here.
In this case Al Awlaki could be indicted and offered terms of surrender. He was well connected, had email, and his whereabouts were perfectly known. A registered mail would be just fine. If he doesn't surrender then he becomes an outlaw, wanted dead or alive. The point is that the choice of surrendering vs. being targeted should belong to the accused.
Another negative aspect of blowing opponents up is that they never get a chance to face their accusers and to counter accusations against them.
Allowing extrajudicial assassinations of criminals inevitably leads to assassinations of political opponents. Criminals are just a trial balloon, to see how the society reacts, and to establish a precedent. Stalin started with show trials in 1930's, but eventually NKVD got so busy that the trial was reduced to mere rubberstamping of prewritten papers. Al Awlaki haven't gotten even that much. His fate is similar to the fate of Leon Trotsky - and nobody can debate that Stalin's orders were not exactly lawful (even though Trotsky was a bloody murderer himself, well deserving all that came his way.) If you give a US President the right to assassinate a US citizen abroad then you give the same right to Joseph Stalin - and to Mahmoud Ahmadinejad, and to Kim Jong-un, and to every lesser dictator. What would you say if King Abdullah orders Huma Abedin killed at 2201 C Street, or in the White House? (Note that it doesn't matter what Ms. Abedin thinks about her citizenship; all that matters here is what the King thinks.)