White House spokesman Carney defended the paper’s definition of imminent threat, saying, “What you have in general with al Qaida senior leadership is a continuing process of plotting against the United States and American citizens. I think that’s fairly irrefutable.”

Carney reiterated that a pillar of the policy is the post-9/11 congressional resolution authorizing the president to use “all necessary military force” to fight al Qaida.

“Certainly under that authority the president acts in the United States’ interest to protect the United States and its citizens from al Qaida,” he said.

The white paper also asserted that judges cannot review or block targeted killing orders because that “would require the court to supervise inherently predictive judgments by the president and his national security advisers as to when and how to use force against a member of an enemy force against which Congress has authorized the use of force.”

The memo said that the legal basis for targeted killing relies on two principles of international law: the right to kill an enemy in the course of an armed conflict and the right of a nation to defend itself against an imminent threat of armed attack.

By citing both, however, the administration is “conflating” distinct legal principles “so that it’s never clear which legal basis the government is relying on,” said Prasow of Human Rights Watch. “It may be able to rely on both, but what it does with the self-defense rationale is that it takes the idea that the threat is imminent and then writes away the definition of imminence.”

“This idea that the government can rewrite legal terms is one that we’ve seen before,” she said, citing the Bush administration legal opinion that redefined torture, allowing the CIA to use interrogation methods such as waterboarding that most experts regard as torture.

Video: Leaked Memo Says US Can Lawfully Kill Overseas