“That record, and Mr. Awlaki’s calls for more attacks, presented Mr. Obama with an urgent question: Could he order the targeted killing of an American citizen, in a country with which the United States was not at war, in secret and without the benefit of a trial? The Justice Department’s Office of Legal Counsel prepared a lengthy memo justifying that extraordinary step, asserting that while the Fifth Amendment’s guarantee of due process applied, it could be satisfied by internal deliberations in the executive branch.”