What circumstances justify the use of military force against an individual?

Civil libertarians are objecting to the U.S. missile attack Friday that killed Anwar al-Awlaki, an American citizen who is believed to have played an important role in Al-Qaida. Authorities believed Al-Awlaki had gone from a propaganda role to being increasingly involved in armed operations for the terrorist group.

Last year, the Obama administration added Al-Awlaki to a list of people the CIA should try to kill or capture, a move that prompted protests from his family and a lawsuit from the ACLU. The suit argued that, as an American, Al-Awlaki was entitled to due process of law before he could be deprived of life or liberty. The suit was dismissed.

As the ACLU pointed out on Friday, Al-Awlaki was killed in Yemen, not on a battlefield. It said the attack amounted to the execution of an American citizen.

Today’s Question: What circumstances justify the use of military force against an individual?