By saying that due process applies to drone strikes on suspected terrorists in places such as Pakistan and Yemen, the administration implicitly concedes that such operations are fundamentally different from shooting an enemy soldier during a battle. In the latter case, both the identity of the enemy and the threat he poses are clear, and so is the argument for self-defense. When it comes to people marked for death by the president, however, all of these issues may be matters of dispute.
During Brennan's confirmation hearing, Dianne Feinstein, the California Democrat who chairs the Senate Intelligence Committee, was at pains to portray Anwar al-Awlaki, one of the three Americans killed by drones so far, as "a senior operational leader" of al-Qaida who posed "an imminent threat" -- the sort of target discussed in the DOJ white paper. Feinstein herself had to testify on these points because neither Brennan nor any other administration official will discuss the evidence against people targeted by drones.
The lack of transparency is especially troubling because the administration's definition of "imminent threat" does not hinge on plans for a specific attack. Furthermore, the white paper explicitly leaves open the possibility that the criteria it describes, while sufficient to justify a presidential death warrant, may not be necessary, and it acknowledges no geographic limit on Obama's license to kill. Brennan conspicuously dodged the question of whether the president can order hits on U.S. soil.
Given this alarming combination of deadliness and silence, it is not hard to see why, as McMahon put it, "some Americans question the power of the executive to make a unilateral and unreviewable decision to kill an American citizen who is not actively engaged in armed combat operations against this country." The real puzzle is why so many Americans seem happy to trust the president with this power.