The very scenario to be guarded against is a president using the pretext of a terrorist attack to seize extraordinary powers. Isn't that among the most likely scenarios for the United States turning into an authoritarian security state? To be sure, if Americans are at the controls of fighter jets en route to Hawaii, of course Obama could order that they be fired upon. If Americans hijacked a plane, of course it would be permissible to kill them before they could crash it into a building. But those are not the sorts of targeted killings that Senator Rand Paul asked about in a letter to White House counterterrorism adviser John Brennan, prompting Holder's response.
If you read to the end of Holder's letter, to the passage where he says, "Were such an emergency to arise, I would examine the particular facts and circumstances before advising the president on the scope of his authority," it becomes clear that, despite invoking Pearl Harbor and 9/11, even he isn't envisioning a response to an attack in process, which would have to happen immediately. So what does he envision? If he can see that a "for example" is necessary to explain, he ought to give us a clarifying example, rather than a nonsensical one that seems to name-check events for their emotional resonance more than for their aptness to the issue.
For the Obama Administration, conceding that the executive branch is legally forbidden to do certain things is verboten, despite the dangers of an unchecked president.
Elsewhere in his letter, Holder writes that "the US government has not carried out drone strikes in the United States and has no intention of doing so. As a policy matter moreover, we reject the use of military force where well-established law enforcement authorities in this country provide the best means for incapacitating a terrorist threat." Interesting they reject it "as a policy matter," but aren't willing to reject military force in the United States as a legal matter, even in instances where law enforcement would better incapacitate the threat. For the Obama Administration, conceding that the executive branch is legally forbidden to do certain things is verboten, despite the fact that an unchecked executive is much more dangerous than the possibility of a future president failing to do enough to fight back against an actual attack on our homeland*.
Any thinking person can see that Holder's letter is non-responsive, evasive, and deliberately manipulative in its sly reassurances, right down to the rhetorically powerful but substantively nonsensical invocation of 9/11. (Being more subtle about it than Rudy Giuliani doesn't make it right.) To credulously accept this sort of response, on an issue as important as this one, is behavior unfit for any citizen of a free country, where safeguarding the rule of law is a civic responsibility.