From the moment the United States began military operations against the Islamic State, questions have arisen about the domestic legal basis for the use of American force. The major debate concerns statutory limits: with narrow exceptions, the Administration has not relied upon Article II alone for legal authority to strike ISIL. President Obama has invoked two statutory sources for his power to act: the 2001 Authorization to Use Military Force (AUMF), targeting al-Qaeda; and the 2002 AUMF for Iraq. The argument for the applicability of these laws to current operations is plausible, though not widely embraced, and remains subject to significant criticism.

The President has accompanied this statutory argument with requests to Congress for a new, ISIL-specific authorization. To date, Congress has not provided one. Arguably, Congress has acquiesced to the Administration’s interpretation of these older statutes—though it retains the power to indicate otherwise. The President appears to have acted with the support of leadership of both parties in both chambers, and Congress has taken no actions to stop or hinder military operations.

In the absence of judicial review, Congress has significant influence over the legitimacy of the Administration’s interpretations. Even its silence can have a major impact on the credibility of a legal argument from the executive branch. Many observers have understood the lack of legislation to date to represent an agreement between the President and Congress to postpone action until after the midterm elections. With nearly a month having passed since polls closed, congressional silence now stands out more prominently, and may soon be nearing its end.

Legislators are now working to craft a new AUMF, specific to ISIL. If passed into law, it will likely be the single most important source for understanding whether Congress has indeed accepted the Administration’s interpretation of the AUMFs—or whether, conversely, the President’s military operations prior to the passage of an ISIL-specific AUMF lacked real authorization from Congress. For this reason, it will be crucial that any new AUMF explicitly address its relationship to these existing authorities.

Legislators and commentators have already reached a consensus to narrow or potentially phase out these previous statutes. Most credible proposals for a new AUMF would sunset the 2001 AUMF, repeal the 2002 AUMF, or both. Proposals from two sets of legal experts—the “Principles” and the draft AUMF at Lawfare—and at least three other legislative proposals from members of Congress—Senator Paul and Representatives Larson and Schiff—would put the 2001 AUMF on a path to gradual sunset. (Or at least its substance: in the case of the Lawfare editors’ proposal, there would be immediate repeal followed by immediate time-limited reauthorization in a separate bill). And the base of support for outright repealing the 2002 AUMF is, if anything, more broad—including all previously mentioned proposals as well as those of Senator Kaine and Representative Issa.

Sunsetting or repealing these statutes will be an important first step; on its own, however, it will be ambiguous whether Congress has accepted the President’s interpretation of them.

To understand why, consider the effect if Congress allowed the status quo to persist—not passing an ISIL-specific AUMF and not sunsetting or repealing present authorities. The case for congressional acquiescence, absent other signals from Congress, would grow stronger every day. Of course, this case would become less clear if Congress signaled its disapproval of military operations, through, for example, holding up nominees or refusing to appropriate funds. Senator Tim Kaine (D-Va.) recently flagged these options in an event at The Wilson Center, noting that “if Congress doesn’t act, at some point there’s going to be those of us who are going to be introducing resolutions of disapproval or trying to get in the way, and stop a war from going on.” Such actions could easily voice disagreement about legal authority without opposing policy—after all, Kaine favors military operations against ISIL, and would authorize them in his own ISIL-specific proposal. Absent these steps, however, the longer operations persisted without pushback, the more Congress’ approval could be ascribed to its silence.

What changes by passing an ISIL-specific AUMF, whether including a sunset, a repeal, or both? One plausible reading of such an action is that it signals nothing at all: no statement for or against the Administration’s interpretations of the 2001 and 2002 AUMFs. Jack Goldsmith notes, in the context of the 2001 AUMF, that

 …[A] sunset on an AUMF will mean, and should signal, only that in our democracy it is prudent that Congress reconvene to assess and update the President’s authorities to use force every few years.

Goldsmith is undoubtedly right about the desirability of recurring input from Congress in debates over the use of force. A requirement for new authorization can do more than embody this value, however: it could assert Congress’ view that prior authorizations were legally insufficient for American military operations against ISIL.

A November Congressional Research Service report on several ISIL-specific AUMF proposals flagged this point:

Given this presidential application of existing AUMFs, the repeal provisions might take on the role of repudiating the President’s positions on his existing authority to use military force against the Islamic State and other terrorist groups. Repeal of these AUMFs could be seen as an indication that Congress disagrees with the President’s interpretation of his existing authorities to use force and that it intends to replace his existing authority with a possibly narrower authority in an IS AUMF.

Whether 2001/2002 AUMF sunset, repeal, or both “could be seen as an indication that Congress disagrees with the President’s interpretation” will vary depending upon how any AUMF bill comes to pass: the language of the legislation, especially any statements of purpose; and any comments in hearings, the floor of Congress, or other public remarks. Senator Rand Paul has offered one example of what this might look like: his proposal for an ISIL-specific AUMF explicitly states that the 2001 AUMF “does not provide any authority for the use of military force against the organization referring to itself as the Islamic State, and shall not be construed as providing such authority.”

Clearly, it is possible for a legislator to support an ISIL AUMF without accepting the Administration’s prior interpretations, and to express this position in a statute (explicitly or implicitly). Indeed, several legislators support operations against ISIL and do not believe the 2001 and 2002 AUMFs provide the basis for conducting them—including, at least, Senators Kaine, Menendez, and Paul. It is therefore easy to envision language in a new AUMF addressing the permissible interpretations of the prior statutes (in addition to sunsetting and/or repealing them). There is even a direct historical precedent: in the 1983 authorization to use force against Lebanon, § 2(a)(5) expressly registered disagreement with President Reagan’s interpretation of the meaning of “hostilities” under the War Powers Resolution.

If Congress does not make clear any disagreement with the Administration’s prior interpretations, it should at least provide a more satisfactory indication of what an ISIL-specific AUMF will achieve. Of course, it will make the Administration happy: the President has repeatedly asked for a new authorization, and has not yet received one. Yet fulfilling a request from the President, in its own right, does not explain the content of what the request entails. After all, the Obama Administration has identified almost nothing it cannot do against ISIL under its existing authorities, and provides consistent updates on operations it claims are proceeding based on current statutes.

Further, absent an intention to correct the Administration’s legal interpretation, the significance of a new ISIL AUMF is not immediately evident. If the Administration had chosen to frame its argument for a new, ISIL-tailored AUMF with an explicit “ask”—some significant objective the United States could not accomplish without Congress weighing back in—then the purpose of a new authorization would be clear. Yet if instead Congress is merely reiterating an authority it already believes the President possesses, through statutes it already passed, a new AUMF has only marginal value for the democratic process. After all, Congress does not “reconvene” to reauthorize symbolically all manner of military or foreign relations decisions (including the placement of American troops in embassies and bases worldwide). One must ask why the need was so pressing in this case, and Congress should answer.

If Congress does not clarify what the new AUMF adds, then it is a credible interpretation that the Administration and Congress alike continue to hold doubts that Congress authorized current operations via the 2001 and 2002 AUMFs. This would at least explain why the President is seeking a new authorization and why Congress is moving toward one.

All told, making sense of any new AUMF will not be possible without an interpretation of the older AUMFs—and on this, Congress should be the decisive voice. It will be crucial that any new AUMF explicitly address its relationship to these existing authorities, over and above specifying the terms of sunset or repeal.

By clarifying what the statutory basis was for military operations prior to the new authorization, in whichever way it chooses to do so, Congress can also send an important medium- and long-term signal. The next President will be likely to have at least the 2001 AUMF in place at the beginning of their term. Others have argued that the President could end-run around various restrictions in an ISIL-specific AUMF (e.g., limits on ground troops) by relying on the 2001 AUMF for whatever period of time the two statutes are both in existence. Squarely addressing the “retrospective” statutory interpretation issue I have raised here may be key to constraining the ability of this or the next President to do so—or, alternately, sanctioning such a maneuver. And in future—and inevitable—disputes over the interpretation of AUMFs drafted by Congress and looked to by the Executive, this episode could help define the range of legitimate legal arguments.

If Congress did express that the 2001 and 2002 AUMFs provided insufficient authority for operations against ISIL, such a view may not only move in a constraining direction. Perhaps counter-intuitively, the episode could come to strengthen the war powers of the President: because statutes did not furnish sufficient authority for several months of active military operations, the President’s only basis for carrying out those actions was Article II, even if he did not claim so at the time. Future Administrations may well choose to make use of this saving construction in defining the outer limits of recognized presidential power on a constitutional level. Thus, even if one would prefer that Congress’ ISIL-specific AUMF constrain presidential freedom to interpret force-authorization statutes, drafters will have to note the possibility that doing so will introduce an expansion of unilateral presidential power.

Less important than making itself clear in any one particular way, however, Congress must recognize that this period of operations will have a legacy, and that its meaning will depend heavily on the details of new authorization. Congress must pay careful attention to what its actions in 2015 signal about its actions in 2001 and 2002. What legislators do will have a significant impact on how this episode of sunset or repeal and reauthorization will function as future precedent—and in historical assessments of the legality of the President’s actions before Congress spoke.