“On September 18 [2001], Congress enacted an Authorization for Use of Military Force (AUMF)–if not a declaration of war in name, a declaration of war in purpose. It pronounced the September 11 attacks ‘grave acts of violence’ that ‘pose an unusual and extraordinary threat to the national security and foreign policy of the United States.’ They justified a military response: ‘Such acts render it both necessary and appropriate that the United States exercise its rights to self-defense and to protect United States citizens both at home and abroad.’ …Congress recognized that ‘the President has authority under the Constitution to take action to deter and prevent acts of international terrorism against the United States.’ ”
– John Yoo, War By Other Means, Pages 11-12