President Barack Obama effectively declared war last week on the so-called Islamic State. That is not, of course, exactly how he put it. What he said was that we will “degrade and ultimately destroy the terrorist group known as ISIL,” which is also known as ISIS or the Islamic State.

Claiming that he is simply continuing to act against terrorists, Obama insists that he does not need explicit approval from Congress but that he would “welcome congressional support.”

Actually, the president absolutely requires explicit congressional authorization, and we, as citizens, should demand that Congress debate and vote on the president’s proposal.

The administration has not yet formally explained its legal reasoning, but it suggests that military action against ISIL is authorized by Congress both by its 2001 vote to authorize the use of military force against al-Qaida and its 2002 vote approving the Iraq war.

Neither argument is persuasive. The 2002 resolution authorized the president to defend the United States “against the continuing threat posed by Iraq.” Whatever “continuing” threat we faced from Iraq ended either with the overthrow of Saddam Hussein or the withdrawal of our forces in 2011.

The 2001 Authorization for the Use of Military Force (AUMF) provides more support for the president’s position. It approved the use of force against “those nations, organizations or persons” that the president determines to have “planned, authorized, committed or aided” the 9/11 attacks.

The relationship between ISIL and al-Qaida, however, is complex. They were originally independent; after the U.S. invasion of Iraq, they were for a time allied; now, they are hostile to one another. Nor has the administration shown that ISIL or any of its current leaders planned, authorized, committed or aided the 9/11 attacks.

Note, however, that the language of the AUMF gives considerable discretion to the president: He doesn’t have to prove to anyone else’s satisfaction that some organization had anything to do with 9/11 in order to use military force against it. If “he determines” that the group aided those attacks, that’s good enough.

It seems that Obama has been comfortable making the determination that any terrorist groups that are now or have ever been “associated with” al-Qaida are covered by the 2001 AUMF.

As Obama said in his 2013 speech to the National Defense University, “the United States is at war with al-Qaida, the Taliban and their associated forces.” That phrase, “associated forces,” as the president used it, seems to include all radical jihadi groups — including ISIL.

As citizens, however, we should not be so comfortable with our president’s having such vast discretion to commit American forces to war. The Constitution gives Congress, not the president, the exclusive authority to declare war for two reasons. Wars can be fought successfully to victory only with the support of the people, and authorization by Congress is a better gauge of popular support than the assertion of a president who will never face another election.

Secondly, wars enable the president and the executive branch to amass powers, at the expense of the legislature and of the people, which is why all friends of republican government must insist on limiting the powers of the presidency. Too many Republicans forgot that lesson during the Bush administration; too many Democrats who insisted on it then seem to have forgotten it now.

Clearly, ISIL represents a new and distinct threat — the prospect of a new, radicalized, expansionist state in the heart of the Middle East. Rather than hiding behind a vote cast 13 years ago in very different circumstances, Obama ought to insist on getting explicit authorization for what will clearly be new action, in a new and complex field of battle.

To be fair to Obama, in that same 2013 speech, he called upon Congress to revisit the AUMF, and the Congress did not act. In its continued silence, he will simply assume its continued acquiescence in his war-making policies — especially if, as seems likely, Congress votes to provide funding for the military actions he is proposing.

While it is tempting to blame Congress for not acting, the fault really lies with us, the citizens. Members of Congress will not vote on war with ISIL unless they believe they are more likely to be voted out of office if they abdicate their constitutional responsibility than if they take a clear stand for or against war with ISIL. So we, as citizens, must demand that Congress fulfill its constitutional responsibility to vote either explicitly to approve this new war, or against it.

Joseph R. Reisert is associate professor of American constitutional law and chairman of the department of government at Colby College in Waterville.

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