This post is the latest installment of our “Monday Reflections” feature, in which a different Just Security editor examines the big stories from the previous week or looks ahead to key developments on the horizon. It is based on remarks delivered by Professor Koh at the  Future of War Conference in Washington, D.C. on February 24, 2015, co-sponsored by the New America Foundation and Arizona State University.

Whatever our differences regarding the future of war, we should all be able to agree on what war must be and what it must not be. Going forward, war must be lawful, but, it must not be perpetual. At some point, it must end.

That brings me to the current debate over the ISIL-AUMF (Authorization of Military Force against the Islamic State), where the Administration is trying to strike a balance between these two goals: (1) Sufficient legal authority: having enough legal authority to effectively fight a potentially long-term battle against the Islamic State, while still taking steps toward (2) Ending the Forever War: not perpetuating America’s Longest War by endlessly expanding the 2001 AUMF, which the President has repeatedly vowed to refine and ultimately repeal. As the President explained in his 2013 speech at the National Defense University, of course, “ [o]ur systematic effort to dismantle terrorist organizations must continue.  But this war, like all wars, must end. …” And so the challenge he posed there was just right: “to engage Congress about the existing Authorization to Use Military Force, or AUMF, to determine how we can continue to fight terrorism without keeping America on a perpetual wartime footing.”

Right now, we are in a rare moment in this town where Congress is seriously considering that question. That is no small feat because it takes so many politicians to tango and because there are very few historical moments (apart from immediately after disasters like Pearl Harbor or 9/11) when a critical mass of elected officials seems ready to put itself on the line by voting to authorize war.

But in this town, at this moment, Congress seems overly fixated on the first goal—sufficient legal authority—while giving short shrift to the second —ending forever war. But on reflection, the second goal is far more important. Why? Because our armed conflict with Al Qaeda, the Taliban and associated forces, which began just after 9/11, is nearly 14 years old: by a wide margin the longest war in American history, 8 years longer than the Revolutionary War, and 10 years longer than either the Civil War or World War II. As the President noted at NDU: “the choices we make about war can impact — in sometimes unintended ways — the openness and freedom on which our way of life depends,” particularly when that war seems to stretch on endlessly.

So our challenge is clear: the President and Congress must enact sufficient authority to win a war that must end. Getting the balance wrong would be disastrous. This legislative moment could become the 21st century’s Gulf of Tonkin Resolution. For if Congress were to overbroadly authorize a fight against ISIL that mutates endlessly to include new enemies, or that does not continue the process of reexamining the continuing need for the 2001 AUMF, we will have tipped the balance toward perpetual war.  

How to prevent this from happening? How should the new AUMF strike this balance between sufficient legal authority and ending the forever war? There are many elements of an ISIL-specific AUMF that we could discuss: geographic scope; limits on ground troops; the definition of associated forces; and reporting requirements. I have joined with several other colleagues to specify what the elements of such an AUMF should be. But at this moment, the crucial legal question is: What will the new ISIL AUMF say about the 2001 AQ AUMF? What is the relationship between these two laws?

Logically, there are two possibilities: addition or substitution? On the one hand, the new ISIL-specific AUMF could simply add to the legal authority to wage war that the President already has under the 2001 AUMF. If you pardon the metaphor, the new ISIL AUMF could provide the President a 3-year belt in addition to the 2001 AUMF “suspenders” he has already has claimed under his controversial reading of the 2001 AUMF as a source of legal authority. I have elsewhere explained why that reading is legally defensible. But even so, it is not clear why either Democrats or Republicans would want to vote for such redundant and additive authority, especially since there has been considerable bipartisan opposition to the President’s theory that the 2001 AUMF applies to ISIL. In his NDU speech, President Obama pointedly promised, “I look forward to engaging Congress and the American people in efforts to refine, and ultimately repeal, the AUMF’s mandate.  And I will not sign laws designed to expand this mandate further.” Yet if he signs an ISIL-specific AUMF, while leaving the 2001 AUMF in full force for all time, wouldn’t he have effectively done that?

Alternatively, the ISIL AUMF could substitute as the sole basis of legal authority where ISIL is concerned. The enactment of an ISIL-specific bill could be read as a substitution or switch, with the narrower, more specific, later-in-time bill becoming the President’s exclusive lawful authority for fighting ISIL. I have heard from some in the Administration that substitution or switching is what they believe this new AUMF would do. But on this critical question, the Administration’s draft is totally silent. For that reason, in the unlikely event that the Administration bill were to pass as offered, it could be read either as addition or substitution. This ambiguity needs to be fixed. The American people are entitled to know whether “2 and 2 means 4” or “2 and 2 means 2.” Under questioning, Secretary Kerry recently testified that the administration would accept supersession language to “live under the confines of what we pass here”:

“If Congress passes a new [ISIL] specific AUMF, we will support the inclusion of language in the new AUMF that will clarify that the [ISIL] specific AUMF rather than the 2001 AUMF is the basis for the use of military force, and I think that will give comfort to a lot of people.”

But if that is what the new AUMF is really intended to do, why not say so clearly? 

So if you take anything away from these remarks, it should be: “supersede and sunset, not silence.” If Congress is concerned about being clear and not ambiguous, why not add explicit language to that effect? To clarify the relationship between the ISIL AUMF being proposed, and the 2001 AUMF that has driven the war against Al Qaeda so far, any new AUMF should incorporate language resembling Sec. 6 of the bill that emerged from the Senate Foreign Relations Committee last December, which simply reads:

“The provisions of this [law] pertaining to the authorization of use of force against [ISIL and presumably its associated forces] shall supersede any preceding authorization for the use of military force.”

In addition, any new AUMF should sunset the 2001 AUMF three years after the date of enactment of the new AUMF, as Section 8 of the SFRC bill does, i.e. sometime in 2018. Since the President proposes sunsetting the ISIL AUMF in 3 years time anyway, that is a guaranteed moment of future legislative consideration. And if Congress is going to have to look at the ISIL picture anyway in 2018 to decide whether to renew the ISIL AUMF, why not look at the full state of our conflict against various terrorist networks at that time?

Let me close by saying that I have yet to hear a good argument against either supersession or sunsetting. Saying that the ISIL AUMF supersedes the 2001 AUMF with respect to ISIL simply makes clear that Congress is voting on something real here, not just giving the President a legal belt to wear on top of his 2001 AUMF suspenders. By accepting supersession language in the bill– as Secretary Kerry said the Administration would–the President would be creating an important precedent. He would be making a major concession to congressional prerogative over warmaking. He would be foreswearing the right to fight ISIL under the kind of broad and vague authority he has invoked for over six months. And he would genuinely depart from the ever-expanding interpretation of the 2001 AUMF that plagued the last administration.

Finally, and most crucially, a sunset is not a repeal; it need not even be read as a proposal to repeal in the future. Just look at the renewals of the Patriot Act. A sunset is simply a shared congressional-executive agreement to reassess the situation together as a nation sometime in 2018, 17 years after 9/11, when we will have a new president and congress, and have a much better sense of whether our war effort against ISIL has made progress.

The fact that we are all currently operating behind John Rawls’ “veil of ignorance”—where we don’t know now who will control the White House, either house of Congress or ISIL at that point—is precisely the reason to call now for a moment of collective reassessment in the future to ensure we talk then about just how long we want the new conflict with ISIL to go on. The fact that this enemy and conflict may morph in unforeseeable ways is precisely the reason that we should include a sunset, because nobody in Congress today can predict what the conflict will entail in 3 years —which would surely be a good time and sound reason to revisit it. A sunset clause is good for Congress because it ensures that Congress is not giving another open-ended check, but has a guaranteed chance to review the situation in the future. Why shouldn’t we want our members of Congress to have a seat at the table as we decide how much war we want to have and if and when American soldiers should be sent into battle?

There are many other elements of an AUMF we could discuss but to me, this is critical question: “How—in this rare moment of legislative consideration—to strike the right balance between sufficient legal authority and ending the forever war?” The answer is simple: “supersede and sunset, but not silence.”