Yoo’s main argument is the theory that the president, as commander-in-chief of the armed forces, has the authority to initiate war with or without congressional authorization, at least if he thinks it is necessary to protect national security. This argument is flawed for reasons I summarized here:
Article I of the Constitution unequivocally gives Congress, not the president, the “power . . . to declare War.” The Founding Fathers sought to avoid a situation where one man had the power to commit the nation to war by himself. Even Alexander Hamilton — the biggest supporter of sweeping presidential power among the framers of the Constitution — recognized that “the Legislature have a right to make war” and that “it is . . . the duty of the Executive to preserve Peace till war is declared…”
In some of his other writings, Yoo has argued that Congress’ power to “declare” war only includes the right to announce that a legal state of war exists, which is distinct from the power to actually initiate fighting. But as legal scholar Michael Ramsey explains, the original meaning of “declare” includes initiation of conflict, not just a legal declaration of its existence.
Yoo further contends that even if Obama does need congressional authorization, he already has it under the 2001 Authorization for the Use of Military Force. But, as I and many others have noted, the 2001 AUMF only covers groups that perpetrated or abetted the 9/11 attacks, and cannot be used to justify attacking a group that did not even exist in 2001 and is today actually an enemy of al Qaeda. Yoo claims that the 2001 AUMF authorizes preemptive and preventive attacks against any terrorist group that might threaten the United States, because it states that “The President has authority under the Constitution to take action to deter and prevent acts of international terrorism against the United States.” But if that passage really gave the president blanket authority to wage war against “international terrorism,” there would have been no need for the more specific authorization to “use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons.” It is a longstanding principle of legal reasoning that we should not interpret laws in such a way as to render large parts of them completely superfluous. It is also worth noting that the passage quoted by Yoo is from the preamble of the AUMF, not the operative part. Preambles are not usually considered legally binding.
Yoo suggests that the use of force against ISIS is authorized by the 2002 Iraq AUMF, which gave President Bush the authority to initiate the Iraq War in 2003. This argument overlooks the fact that the 2002 AUMF is limited to the use of force against “the continuing threat” posed by Iraq. For reasons I outlined here, this is best interpreted as limited to the threat posed by the regime of Saddam Hussein and its allies. As Yoo notes, the 2002 AUMF give the president the authority to enforce relevant UN Security Council resolutions on Iraq. But the resolutions in question (summarized in this 2004 article by Yoo), all target the Iraqi government, not insurgent and terrorist groups.
Michael Ramsey offers a stronger version of the claim that the 2002 AUMF authorizes Obama’s actions. He notes that ISIS is in many ways a successor organization to some of the Sunni insurgents that US forces battled in Iraq in 2007-08. If the 2002 AUMF covers the fighting between US forces and ISIS’ predecessors back then, why not also Obama’s actions today? The answer, I think, is that the 2007-08 fighting was legally authorized not by the 2002 AUMF but because US forces had actually been attacked by the Sunni insurgents. US troops don’t need congressional authorization to defend themselves. But the US combat role in that conflict ended when American troops were withdrawn in 2011. President Obama himself has repeatedly said so. Therefore, today’s intervention against ISIS cannot be justified as a continuation of the fighting against Sunni insurgents several years ago. It is a new war and requires new congressional authorization.