As Bob Levy has already ably probed the legal issues surrounding the killing of Anwar al-Awlaki, I'll just append a few miscellaneous thoughts.
First, over the last decade we have been repeatedly told by foreign policy hawks that it is foolish, and even borderline offensive, to suggest that aggressive U.S. action abroad may have the counterproductive and unintended consequence of swelling the ranks of terror groups. When evaluating the wisdom of drone strikes or invasions of other countries, we need not even factor in the downside risk of "blowback" stemming from such actions, because "they hate us for our freedoms." In other words, radical Islamist terrorists are fundamentally motivated by a vision of a global caliphate, not by any grievances stemming from real or perceived injuries inflicted by U.S. policy. I think of this as the "No Marginal Terrorist" Theory, because it posits that people are motivated to join terror groups strictly for reasons connected with either personal psychology or theology, such that reactions to specific U.S. actions never make the difference at the margin.
At the same time—and often by the same people—we are told that Anwar al-Awlaki posed a grave threat to the United States, not so much because of any particular logistical genius he possessed, but because he was so dangerously effective as a recruiter and propagandist who could inspire people already living in the West to jihad. Surely, then, it's relevant to inquire into the nature of this lethally effective propaganda. Here is an excerpt from what The Guardian calls one of "his most direct, English-language statements endorsing terror attacks on Americans":
With the American invasion of Iraq and continued U.S. aggression against Muslims, I could not reconcile between living in the U.S. and being a Muslim, and I eventually came to the conclusion that jihad against America is binding upon myself just as it is binding on every other Muslim....
To the Muslims in America, I have this to say: How can your conscience allow you to live in peaceful coexistence with a nation that is responsible for the tyranny and crimes committed against your own brothers and sisters?
Possibly al-Awlaki is just a sort of Salafist James Earl Jones, and the sheer hypnotic beauty of his voice is what compels people to sacrifice their lives for him, without regard to the specific contents of his sermons. Still, it seems to be a problem for the No Marginal Terrorist Theory if a propagandist who was believed to be uniquely effective at motivating people to become terrorists used rhetoric like this to do it.
Second, a good deal of the coverage I've been seeing has treated the conclusions of U.S. intelligence analysts about al-Awlaki's role and status within al Qaeda in the Arabian Peninsula (AQAP) as ironclad facts rather than contestable inferences from necessarily patchy data—even though the past decade should have made it abundantly clear that analysts sometimes get it wrong. Certainly al-Awlaki is no "innocent" in any sense of the word, but on the crucial claim that he'd progressed from terrorist mascot to mastermind, it's worth noticing how much of the case depends on plots that the cleric was "linked to" or "believed to have had a hand in planning." At least one Yemen expert has argued that al-Awlaki's status within AQAP has been wildly inflated, describing him as a "midlevel religious functionary."
While there is some public evidence that certainly seems to support the conclusion that al-Awlaki had gone "operational"—that he did not merely advocate jihad in principle, but played a key role in planning and directing terrorist acts—the bulk of it remains classified. As we learned to our great cost after the invasion of Iraq, a top secret clearance does not actually grant omniscience, and sometimes a case that seems like a slam-dunk on the surface falls apart under impartial scrutiny. Paradoxically, the administration's refusal to submit to that scrutiny seems to have given its determinations an aura of oracular certainty.
Third, the case for targeted killing here relies very heavily on the fact that al-Awlaki had put himself beyond the reach of feasible arrest. The most ardent hawk would recoil at the prospect of simply dropping a bomb on a citizen suspected of al Qaeda ties in New Jersey, or London. But as Robert Farley notes, what is "feasible" is at least in part a matter of judgments about the risks and benefits of attempting a capture. So we're required to entrust to the executive branch to determine not just when a particular citizen has joined the enemy, but under what conditions it's worth the risk of attempting to take them alive.
In al-Awlaki's case, one can at least say—as the judge who rejected a lawsuit brought by his father did—that the target was plainly aware the government was after him, and in theory could have offered to surrender himself if he'd been interested in seeking his day in court. (I stress "in theory" because it's hard to imagine AQAP looking favorably on such a decision in the wildly improbable event al-Awlaki had been inclined to make it.)
But remember that this was supposed to be a wholly covert operation, and would (according to the administration) imperil national security if discussed in any way—even though the national security risk appears to have diminished a great deal now that it's a matter of taking credit rather than blocking litigation. There was an advance leak in this instance, but the next citizen on the list may have no idea there's a Hellfire missile with his name on it. What we think about the specific instance of al-Awlaki, then, seems less important than how we feel about a case in which everything goes according to plan. That is, an American citizen is simply killed abroad with no advance warning, on the basis of an executive determination that he has joined an enemy power and poses an imminent threat, and no guarantee that the United States will acknowledge (let alone justify) the operation even after the fact.
Fourth and finally, the debate after the fact has been a reminder of how utterly useless conventional war metaphors are for grappling with the unique problems presented by the present conflict. Anyone who imagines the very thorny issues presented in the current case are somehow illuminated by analogies from World War II is just kidding themselves: if this conflict were not so plainly unlike World War II and other conventional conflicts between nation states, on so many salient dimensions—if we could straightforwardly treat an ever-shifting array of emerging terror groups as equivalent to a sovereign country's uniformed military—everything would be a good deal simpler.