Disclosure: I am not a lawyer. OK, that's out of the way.
I understand the post by streiff regarding the U.S. killing of American-born Islamist terrorist Anwar al-Awlaki. Like streiff, I was glad President Obama ordered this action. However, I disagree with the position streiff and others have taken in believing Awlaki's killing was unprecedented. In the post, streiff includes two Civil War Confederate guerillas, Marcellus Clark and Champ Ferguson, who were captured, tried, and executed for what the Union called war crimes. As much as I see streiff as an excellent historian and enjoy reading streiff's posts on RedState, there are two others Civil War rebels missing from the post. The two had more in common with Awlaki (along with Clark and Ferguson) than with those regular soldiers and militia fighting for the Confederacy. They are William Quantrill and "Bloody Bill" Anderson.
Both men were guerillas based out of the border state of Missouri. In fact, Anderson served under Quantrill for some time until the two had a falling out and the outfit was split between the two. While they were together, Quantrill and Anderson, along with some men who would later emerge as members of the James-Younger outlaw gang, perpetrated the massacre of civilians in Lawrence, Kansas. With this and similar actions orchestrated by the two, which I believe can only be thought of as acts of terrorism, neither Quantrill or Anderson were considered lawful combatants by either the Union or the Confederates.
Based on my limited readings, it doesn't appear that the Union had any interest in bringing Quantrill and Anderson alive. In 1864, while the war was still going on, Anderson was lured into an ambush by Union forces intent on killing him, which did occur. Quantrill, on the other hand, was still on the loose after the war was more less over following the surrenders of Lee in Virginia and Johnston in North Carolina. While in Kentucky, Quantrill was also led into an ambush and shot, dying of his wounds nearly a month later.
As far as I know, there was never any attempt to have either arrested and tried for their crimes, nor any indication of regular due process being used that authorized the deaths of these two men. They were active in fighting against the United States, doing so in an illegal manner, just as Awlaki had done. So I believe there was precedent for Obama's ordering the death of the Islamist terrorist. And when it comes to American law, precedence matters a great deal. Even modern courts regularly cite as precedent various 19th Century decisions such as Gibbons v. Ogden (which is being used by the government for its Obamacare defense), Ex parte Milligan (noted in Hamdan), and to regularly justify judicial review by citing Marbury v. Madison. Even the methods used to kill Anderson and Quantrill as compared to Awlaki differ only in respect to the technology available at the times these people died; in my opinion, it's a non sequitur.
So as far as I'm concerned, there was legal precedent for Obama's ordering the death of Anwar al-Awlaki.
Cross-posted at Scipio the Metalcon.