Yes, the president does have the authority to use military force against American citizens on US soil—but only in "an extraordinary circumstance," Attorney General Eric Holder said in a letter to Senator Rand Paul (R-Ky.) Tuesday.
Last month, Paul threatened to filibuster the nomination of John Brennan, Obama's pick to head the CIA, "until he answers the question of whether or not the President can kill American citizens through the drone strike program on U.S. soil." Tuesday, Brennan told Paul that "the agency I have been nominated to lead does not conduct lethal operations inside the United States—nor does it have any authority to do so." Brennan said that the Justice Department would answer Paul's question about whether Americans could be targeted for lethal strikes on US soil.
Holder's answer was more detailed, however, stating that under certain circumstances, the president would have the authority to order lethal attacks on American citizens. The two possible examples of such "extraordinary" circumstances were the attack on Pearl Harbor and the 9/11 terrorist attacks. An American president order the use of lethal military force inside the US is "entirely hypothetical, unlikely to occur, and one we hope no president will ever have to confront," Holder wrote. Here's the bulk of the letter:
As members of this administration have previously indicated, 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. We have a long history of using the criminal justice system to incapacitate individuals located in our country who pose a threat to the United States and its interests abroad. Hundreds of individuals have been arrested and convicted of terrorism-related offenses in our federal courts.
The question you have posed is therefore entirely hypothetical, unlikely to occur, and one we hope no president will ever have to confront. It is possible, I suppose, to imagine an extraordinary circumstance in which it would be necessary and appropriate under the Constitution and applicable laws of the United States for the President to authorize the military to use lethal force within the territory of the United States. For example, the president could conceivably have no choice but to authorize the military to use such force if necessary to protect the homeland in the circumstances like a catastrophic attack like the ones suffered on December 7, 1941, and September 11, 2001.In a Google+ Hangout last month, President Obama refused to say directly if he had the authority to use lethal force against US citizens. As Mother Jones reported at the time, the reason the president was being so coy is that the answer was likely yes. Now we know that's exactly what was happening.
"Any use of drone strikes or other pre-meditated lethal force inside the United States would raise grave legal and ethical concerns," says Raha Wala, an attorney with Human Rights First. "There should be equal concern about using force overseas."