In his speech at Northwestern University yesterday (as Lucy Steigerwald noted), Attorney General Eric Holder said the "due process" required before killing suspected terrorists in other countries, including American citizens, is not the same as the due process required for criminal defendants in the United States. He laid out three sufficient (but perhaps not necessary) conditions for the targeted killing of "a U.S. citizen who is a senior operational leader of al Qaeda or associated forces, and who is actively engaged in planning to kill Americans" (as Anwar al-Awlaki was alleged to be):

First, the U.S. government has determined, after a thorough and careful review, that the individual poses an imminent threat of violent attack against the United States; second, capture is not feasible; and third, the operation would be conducted in a manner consistent with applicable law of war principles.

The first condition presupposes that the target is indeed "a senior operational leader of al Qaeda or associated forces," which is the sort of thing the government would have to prove in a criminal trial. As for the imminence of the threat posed by the alleged terrorist, the "thorough and careful review" to which Holder refers is carried out entirely by the executive branch, with no role for the judiciary or the legislature (although "the Executive Branch regularly informs the appropriate members of Congress about our counterterrorism activities, including the legal framework, and would of course follow the same practice where lethal force is used against United States citizens"—after the fact, apparently). The feasibility of capture likewise is determined entirely by the president and his men. That condition (assuming it is necessary) seems to rule out targeted killings within this country, where capture presumably would be feasible. Otherwise there do not seem to be any geographical limits:

Our legal authority is not limited to the battlefields in Afghanistan. Indeed, neither Congress nor our federal courts has limited the geographic scope of our ability to use force to the current conflict in Afghanistan. We are at war with a stateless enemy, prone to shifting operations from country to country. Over the last three years alone, al Qaeda and its associates have directed several attacks – fortunately, unsuccessful – against us from countries other than Afghanistan. Our government has both a responsibility and a right to protect this nation and its people from such threats. 

This does not mean that we can use military force whenever or wherever we want. International legal principles, including respect for another nation’s sovereignty, constrain our ability to act unilaterally. But the use of force in foreign territory would be consistent with these international legal principles if conducted, for example, with the consent of the nation involved – or after a determination that the nation is unable or unwilling to deal effectively with a threat to the United States.

In short, Holder claims that Congress, by authorizing the use of military force against those responsible for the 9/11 attacks, empowered the president to order the execution of anyone he identifies as a terrorist, wherever that person may be found (with the possible exception of the United States). If presidents were infallible and always virtuous, there would be no problem with this policy; since they are neither, we should perhaps be wary of letting them decide exactly how much process is due for those they deem deserving of death.

More on the president's license to kill here.