"It may not always be feasible to capture a United States citizen terrorist who presents an imminent threat of violent attack," Holder said during a speech at the Northwestern School of Law in Chicago. "In that case, our government has the clear authority to defend the United States with lethal force."
He said the use of such deadly force "will comply with the four fundamental law of war principles."
"The principle of necessity requires that the target have definite military value," he said. "The principle of distinction requires that only lawful targets -- such as combatants, civilians directly participating in hostilities, and military objectives -- may be targeted intentionally. Under the principle of proportionality, the anticipated collateral damage must not be excessive in relation to the anticipated military advantage. Finally, the principle of humanity requires us to use weapons that will not inflict unnecessary suffering.
"These principles do not forbid the use of stealth or technologically advanced weapons. In fact, the use of advanced weapons may help to ensure that the best intelligence is available for planning and carrying out operations, and that the risk of civilian casualties can be minimized or avoided altogether."
As for the extra-judicial nature of some of the use of lethal force, Holder said, the Constitution guarantees "due process," not "judicial process."
The administration has taken criticism from civil liberty advocates for the killings of al-Qaida founder Osama bin Laden in Pakistan and U.S.-born Anwar al-Awlaki in Yemen.
"This principle has long been established under both U.S. and international law," Holder said in prepared remarks. "In response to the attacks perpetrated -- and the continuing threat posed -- by al-Qaida, the Taliban, and associated forces, Congress has authorized the president to use all necessary and appropriate force against those groups. Because the United States is in an armed conflict, we are authorized to take action against enemy belligerents under international law. The Constitution empowers the president to protect the nation from any imminent threat of violent attack. And international law recognizes the inherent right of national self-defense. None of this is changed by the fact that we are not in a conventional war.
"Our legal authority is not limited to the battlefields in Afghanistan."