Kenneth Anderson, an international law scholar at American University's Washington College of Law, said
U.S. citizens who take up arms with an enemy force have been considered legitimate targets through two world wars, even if they are outside what is traditionally considered the battlefield.
"
Where hostiles go, there is the possibility of hostilities. The U.S. has never accepted the proposition that if you leave the active battlefield, suddenly you are no longer targetable," Anderson said.
In early 2010, the director of national intelligence, Dennis Blair, told a congressional hearing that the U.S. was prepared to kill Americans affiliated with al-Qaida, without mentioning al-Awlaki by name.
"If we think that direct action will involve killing an American, we get specific permission to do that," by which he meant authority from the executive branch, not the courts.
Blair said the military and intelligence agencies had
authority to kill U.S. citizens abroad who were engaged in terrorism if their activities threatened Americans. Since then, U.S. officials have said that al-Awlaki's role in al-Qaida in the Arabian Peninsula (AQAP) had shifted from propagandist to operational tactician and strategist.
The State Department's senior legal adviser, Harold Koh, plainly stated last year the Obama administration's view that it had authority to undertake drone attacks in countries where al-Qaida operatives were located.
Open Channel - Can U.S. legally kill a citizen overseas without due process?
I'm not parroting anything... 
