Attorney General Eric Holder outlined the United States' legal defense of using lethal force against U.S. citizens overseas if that citizen is posing a terrorist threat.  Holder's speech, delivered Monday afternoon at Northwestern University, argued in part that the U.S. Constitution's definition of due process defends the use of lethal force, even without the written consent of the president. Until now, no legal defense was given for the U.S. mission in Yemen which killed al-Qaeda's leading figure Anwar al-Awlaki. Al-Awlaki, who was born in the US, was the radical cleric who successfully took al-Qaeda's message to YouTube. To better understand what's at stake both domestically and internationally we're speaking with two legal experts. Mary Ellen O'Connell is a professor of international law at the University of Notre Dame and vice president of the American Society of International Law and Karen Greenberg is the director of the Center on National Security at Fordham Law School.