Was the killing of Anwar al-Alwaki legal? There is a well accepted legal basis for military action, and we can examine this event and see if it meets those criteria. First, as a part of prosecuting a war our government has legitimately killed American citizens many times. In World War II, American citizens were in the military forces of Italy, Japan, and Germany and they, most in recognized enemy military units, were killed in routine military operations.
Let us not forget that in military operations, civilians are too often killed, U.S. civilians were killed by the Japanese attacks on Pearl Harbor, Hawaii for example. America and its allies also killed civilians - our bombing of Japan killed many Japanese who were not in military units, and we probably accidentally killed citizens of various countries -- even our own who were prisoners of war. This was justified since the Japanese has distributed production of military hardware and we had to eliminate the means of production to end the war.
This is not unusual, we normally try to minimize death and destruction even though our enemies usually do not design their operations to avoid killing civilians who happened to be near targets. So in classic warfare, where uniformed military units are operating, so called collateral damage is an unfortunate fact of life. Citizens of various countries are killed -- some because they are in an opposing force and some because they are in the wrong place at the wrong time.
In the 1950s, guerilla war became more recognized, though it has been around for centuries. We used it in our fight for independence from Britain; many conflicts have taken place where fighters on various sides do not wear uniforms and often do not have formal military training. So the fact that a member of an opposing military is not in a recognized uniform is irrelevant. The people managing attacks against the US are legitimate members of an opposing military force.
So is what we are conducting a legitimate campaign? It certainly is, it is a reaction to when a civilian target (the World Trade Center) was attacked with civilian aircraft by people who appeared to be civilians. The people killed certainly would agree that it is war. In this type of warfare, our opponents do not wear a uniform, they conceal themselves among civilians, and they are difficult to uniquely distinguish. Are they legitimate targets, are they members of an opposing military? Yes. Most of them do take recognizable military training and conduct recognizable military operations at some point. Their leaders facilitate that training, so they qualify as an insurgent government and so are legitimate targets.From a military point of view, the difficulty is that our legitimacy derives from legal statements from our government that have become out of date. Military people cannot determine their own targets and destroy them, without legal authorization from a Constitutional authority. The authorization we are using right now is the "Authorization for Use of Military Force" dated September 18, 2001, known as Public Law 107-40 [S. J. RES. 23]. This specifically mentions the attacks on the World Trade Center and the people who "harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons." Now, Anwar al-Alwaki seems to have communicated with two of the hijackers, but it is a stretch to attack him under this public law. He was a legal target, but we need to update the legal authorization to pursue his colleagues.
Charles Phillips was an Air Force officer from 1978 until he retired in 2005 (working in space, communications, and maintenance), first in the Active Duty for ten years, then in the Texas Air National Guard for ten years, and last in the Air Force Reserve for eight years. He has been a writer all of that time. Now he finds the stories that people are interested in but might have been missed by other reporters.