In his second inaugural address, President Barack Obama declared that “a decade of war is now ending.” White House press secretary Jay Carney later said there was “no question” that the U.S. conflict with al-Qaida was “entering a new phase.”

That day in Yemen, a U.S. drone strike reportedly killed three suspected al-Qaida militants. It was one of several strikes there that week and followed a spate of them in Pakistan. Outgoing Secretary of Defense Leon Panetta said this weekend that drone strikes “ought to continue to be a tool we ought to use where necessary.”

Like the war in Afghanistan, these and hundreds of other drone strikes have occurred under the authority of a concise law passed one week after 9/11. It reads:

That the President is authorized 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, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.

That law – known as the 2001 Authorization for Use of Military Force, or AUMF – is now more than 11 years old. Will it cover this “new phase” of war?

Obama, like President George W. Bush before him, has claimed that the 2001 authorization is the domestic legal basis of the authority to kill and detain not onlymembers of al-Qaida and the Taliban in Afghanistan but also their “associated forces.” Courts have largely agreed with that interpretation, and in 2011 Congress codified it in authorizing military detention.

A Justice Department memo – published Monday by NBC News – repeatedly cites Congress’ authorization in laying out the case for targeting a U.S. citizen “who is a senior operational leader of al-Qaida or an associated force.”

Officials note the AUMF does not have a geographic boundary. Individuals far from the “hot” battlefield in Afghanistan, officials have argued, can still be said to be engaged in an armed conflict with the U.S.

But legal scholars say the AUMF’s authority to detain and kill militants may be undermined if there is no “core” al-Qaida group to speak of, or when active conflict in Afghanistan ends. It may also falter when it isn’t clear exactly how a group or individual is tied to al-Qaida – such as in the web of militant and extremist groups operating in Africa and elsewhere that may claim an affiliation or be ideologically aligned.

“There’s room for shoe-horning them into the AUMF,” says Robert Chesney, a professor at University of Texas School of Law. “But any honest assessment has to concede it’s not obvious that all the more loosely affiliated groups are encompassed.”

The AUMF doesn’t include an expiration date. But the law does have its limits, says Chesney. “It’s not claiming an armed conflict with all terrorism, but with al-Qaida and its associated forces. In theory, there can come an end.”

Last November, shortly before he stepped down as the Pentagon’s general counsel, Jeh Johnson gave a speech on that end. He spoke of a “tipping point,” when the U.S. counterterrorism efforts “should no longer be considered an ‘armed conflict’ against al-Qaida and its associated forces.” Counterterror efforts would then be aimed against individuals and could be handled primarily by law enforcement.

Johnson conceded it was hard to imagine that tipping point. There would be no “peace treaty” to mark it, he said, and he could “offer no prediction about when this conflict will end.”

A preview of the dilemma came in 2011, when the U.S. indicted a Somali man named Ahmed Abdulkadir Warsame in federal court in New York. Warsame was a member of Al-Shabaab, a group in Somalia, and had ties to al-Qaida in the Arabian Peninsula, but he was not connected to any plot against the U.S. He had initially been held by the military, but according to Newsweek reporter Daniel Klaidman, the Obama administration was unsure where he fit under the law.

Jack Goldsmith, a professor at Harvard Law School and former head of the Office of Legal Counsel for President Bush in 2003 and 2004, says “the AUMF is losing its efficacy. We’re in a place when we’re engaged in types of warfare that the nation hasn’t openly debated.”

The “shoehorn” approach may eventually run into legal gray area. Chesney points out that court decisions upholding military detention have generally been linked in some way to the conflict in Afghanistan. (So far, U.S. courts have not taken up lawsuits challenging targeted killing.)

“When the war in Afghanistan ends, and if core al-Qaida is decimated, how do we define who we are at war with?” says Hina Shamsi, director of the National Security Project at the American Civil Liberties Union.

Shamsi argues that the Obama administration is already relying on an overbroad interpretation of the AUMF to justify strikes against alleged militants in Yemen or Somalia without demonstrating precisely how they are associated with al-Qaida or engaged in anti-U.S. hostilities.

Militant groups have emerged as a threat in North Africa – some claiming an affiliation with al-Qaida. The degree to which those groups are plotting against the U.S. or interested in regional control is still being debated. The U.S. is expanding its presence in the region, butat least initially, the government says it is bolstering surveillance and training and assistance for local governments, not taking military action.

A Pentagon spokesman said last week he was “unaware of any specific or credible information at this time that points to an [al-Qaida in the Islamic Maghreb] threat against the homeland, but, again, I’m not ruling it out.”

The U.S. has provided refueling and cargo planes to assist the French intervention in Mali. That is lawful because France is acting “in response to a request for assistance from the Malian government,” Tommy Vietor, a spokesman for the National Security Council, told ProPublica.

Administration officials say strikes against al-Qaida and associated forces are permitted under international law on the basis of self-defense, in addition to the authority the AUMF provides under domestic law. The U.N. has been investigating targeted killings and civilian casualties from drone strikes.

In a case where the 2001 AUMF did not apply, the administration could seek a new authorization from Congress or rely on presidential powers to use force against an imminent threat.

Gen. Carter Ham, the head of U.S. Africa Command, said in an interview with The Wall Street Journal in December that an authorization to address new threats in North Africa was a “worthy discussion.” But what form that would take is unclear. The Pentagon and White House did not comment to ProPublica on the possibility of a new AUMF.

Presidents have used force without Congressional authorization by invoking presidential powers under Article II of the Constitution.

Obama ordered airstrikes over Libya in the spring of 2011 citing international cooperation and “national interest” as justification. (Several lawmakers subsequently sued the administration for bypassing them, but the case was dismissed.) He has also claimed authority to launch pre-emptive cyberattacks, the New York Times reported this weekend. President Bill Clinton cited the nation’s right to self-defense when he bombed Afghanistan and Sudan in 1998 in retaliation for the bombing of U.S. embassies in East Africa.

Obama officials regularly cite self-defense alongside the AUMF in justifying targeted killing. White House counterterror adviser John Brennan has said that the U.S. uses “a flexible understanding of ‘imminence’ ” in determining what constitutes a threat. The Justice Department memo on targeting U.S. citizens also references a “broader concept of imminence,” which it holds “does not require the United States to have clear evidence that a specific attack on U.S. persons and interests will take place in the immediate future.”

Shamsi and other critics of the drone war have noted that some strikes in Yemen in particular appear to target insurgents acting against local government. The U.S. almost never acknowledges particular strikes or details the specific threat posed by an individual.

Johnson, the former Pentagon counsel, told The Wall Street Journal that “the president always has the constitutional authority to protect the nation and important national interests by responding to individual terrorist threats, militarily or otherwise.”

Johnson noted that, for a “sustained armed conflict, the preference should be Congressional authorization.”