THERE IS a contradiction at the center of the counterterrorism policy announced by President Obama in a speech last week. In describing a sharply reduced threat around the world, the president declared that “this war, like all wars, must end.” He promised “efforts to refine and ultimately repeal” the congressional declaration authorizing military action against al-Qaeda and the Taliban and said he would reject any legislation “designed to expand this mandate further.”

At the same time, Mr. Obama made clear that the war as it has been waged outside of Afghanistan and neighboring Pakistan will continue, that it may be conducted in many parts of the world and that it will likely involve more drone attacks. He predicted “a series of persistent, targeted efforts to dismantle specific networks of violent extremists that threaten America.” He said “the detention and prosecution of terrorists” was not always possible and that neither “waiting for attacks to occur” nor “a sole reliance on law enforcement” is sufficient.

So whether Mr. Obama declares the conflict with al-Qaeda over and Congress repeals its authorization or they do not, the fight will continue. How much it will resemble that of the past four years — in which the CIA and military forces conducted hundreds of drone strikes in Pakistan, Somalia and Yemen and supported armed operations by allies in several other countries — will depend more on the evolution of Islamist terrorism than on Washington. If al-Qaeda affiliates continue to proliferate, as they recently have in North Africa and Syria, the conflict may be relatively intense.

So why repeal the Authorization for Use of Military Force (AUMF), the September 2001 legislation the president cited? Mr. Obama worries about “keeping America on a perpetual wartime footing”; he says that “unless we discipline our thinking and our actions, we may be drawn into more wars we don’t need to fight, or continue to grant presidents unbounded powers more suited for traditional armed conflicts between nation-states.”

Those are legitimate concerns; we have argued for more transparency, congressional oversight and executive-branch accountability in the fight against al-Qaeda, particularly when it comes to the use of drones. Yet there’s a danger that dropping the AUMF — as opposed to tailoring it to the new conditions Mr. Obama described — will result in less restraint on presidential power, not more.

Mr. Obama didn’t delve into legal details in his speech. But the top legal advisers at the State and Defense departments during his first term have publicly argued that if the AUMF no longer applies, military attacks on terrorists can still be carried out under Article II of the Constitution, which grants the president power to defend the country against imminent attack. Most legal experts agree with that view. But as Harvard professor Jack Goldsmith argued in a recent blog post, for Mr. Obama or future presidents to conduct military operations on the scale of those now underway outside Afghanistan under Article II — as opposed to an act of Congress — “would be an unprecedented expansion of [presidential] authority.”

Mr. Obama or a future president could, of course, seek specific congressional authorization to begin operations against a new terrorist threat. But another approach would be a proposal by Mr. Goldsmith and several other legal experts to revise the AUMF so as to grant the president authority to pursue al-Qaeda spinoffs, subject to congressional review, while enacting legal constraints on the use of force outside conventional battlefields as well as reporting requirements to Congress. Mr. Obama has adopted some of those strictures in a presidential policy guidance and said all drone strikes were being reported to congressional committees. But these policies could be revoked with the stroke of a presidential pen.

Repeal of the AUMF would force the closure of the Guantanamo Bay prison, something we have supported. But it would also complicate the future military detention of terror suspects — such as the Obama administration’s holding of a Somali militant on a U.S. warship prior to his transfer to the civilian justice system. While Mr. Obama insists that he prefers capturing to killing terror suspects, there have been few such arrests on his watch. A modifed military detention system with appropriate checks and balances could make them more possible.

Mr. Obama pledged to “engage Congress about the existing AUMF to determine how we can continue to fight terrorism” without a permanent state of war. That’s the right discussion, but the result should be neither a legal vacuum nor the declaration of the end of a conflict that, by the president’s own account, is far from over.