The Obama administration is engaged in a debate about the extent of the president’s powers to use lethal force against terrorist organizations, and the deliberations have been accelerated by al-Qaeda’s recent decision to sever ties with a violent Islamist group in Syria.

The focus of internal discussions is whether a law giving the president authority to attack al-Qaeda affiliates still applies to the Islamic State of Iraq and Syria (ISIS), the group that was disavowed last week.

Current and former U.S. intelligence officials said last week’s expulsion marked the first time al-Qaeda had ejected a group that had formally joined its fold, a potentially risky move at a time when the terrorist network’s affiliates have largely eclipsed the core group in strength and relevance.

According to some administration lawyers and intelligence officials, the expulsion of ISIS removes the group from the short list of al-Qaeda “associates” that the president has virtually unlimited powers to strike under a law passed days after the Sept. 11, 2001, terrorist attacks. The group has emerged over the past year as a ruthless player in the Syrian civil war and one whose ability to project its power — and recruits — beyond the immediate battlefield is worrying Western intelligence agencies. On Monday, more than 20 ISIS recruits were killed when a bomb exploded during a class on suicide bombings, Iraqi officials said.

The unofficial list of al-Qaeda affiliates is now down to four: the powerful offshoots in Yemen and North Africa, Somalia-based al-Shabab, and Jabhat al-Nusra, an ISIS rival within the Syrian opposition to President Bashar al­-Assad.

The state of al-Qaeda affiliates

Others think ISIS can still be targeted because of its long-standing al-Qaeda ties and parallel ambitions. Officials who spoke about intelligence matters on the condition of anonymity stressed that the administration has not made any ruling on the subject.

What appears at first glance to be a legal debate over an arcane dispute among terrorists is largely theoretical, since the administration has no current intention of attacking in Syria or Iraq. But the answer has major implications for a far more important issue — what to do about the post-Sept. 11 law authorizing the use of force as the Afghanistan war winds down and an increasingly decentralized al-Qaeda becomes more of a shared ideology than an organized hierarchy.

In recent years, as new groups have posed threats to the United States and to its overseas personnel, the administration has adopted an increasingly elastic definition of the 2001 Authorization for the Use of Military Force, or AUMF, to include groups it says are associated with the al-Qaeda and Taliban organizations explicitly covered by the law.

In a major national security speech in May, however, President Obama said the time had come to “recognize that the threat has shifted and evolved from the one that came to our shores on 9/11.” He said he wanted to “refine and ultimately repeal” the AUMF and would work with Congress to come up with an alternative better suited to the current reality.

Since then, however, most of the administration’s national security attention has been taken up by the leaks of former National Security Agency contractor Edward Snowden, and the subject has languished.

“The default position is to do nothing,” said Rep. Adam B. Schiff (D-Calif.), a member of the House Intelligence Committee who last year introduced legislation to sunset the law with the upcoming end of the Afghan war.

Continuing to rely on the AUMF as the legal justification for counterterrorism operations “becomes increasingly strained when you’re not using it in the theater of war, you’re not using it against organizations that existed on 9/11,” and one of the most violent groups “is not even affiliated with al-Qaeda anymore,” Schiff said in an interview.

The ISIS issue “rekindles the whole question” of revising the AUMF, one U.S. official said.

“You can’t predict the future,” said Seth Jones, an analyst with the Rand Corp. who has advised the military at home and in Afghanistan. “What if the situation were to get notably worse not just in Syria but in Iraq, and we were to see [ISIS] get involved in plotting against the U.S. homeland?”

The administration has other alternatives for using force outside war zones, including the president’s constitutional authority and international self-defense doctrines. But Obama remains leery of justifying drone strikes and other types of military action with constitutional powers he accused his predecessor, George W. Bush, of overusing.

And the more time that passes since Sept. 11, 2001, the more critics of U.S. lethal policies, including some leading allies, are likely to challenge the justification for unilateral strikes and Special Operations actions.

The Pentagon has said it sees no need to change the law, and its press secretary, Rear Adm. John Kirby, said the military “currently has the necessary authority, under domestic and international law, to meet the threat posed by al-Qaeda and other terrorist organizations.”

In last year’s speech, Obama noted that the United States cannot be on a perpetual war footing and “made clear . . . that he will not sign any law designed to expand” the AUMF mandate, said National Security Council spokeswoman Caitlin Hayden.

But it is hard to see how the law can be adapted without expanding its scope.

In a House hearing last week, Jones and other experts offered suggestions on a variety of possible fixes, most of them focused on terrorist goals, operations and capabilities rather than on group affiliations or rhetoric.

“The goal here is not to narrow the scope of the AUMF or to expand the scope of the AUMF, but to align our ways, means and laws in such a way that we can achieve the ends that we’re after,” Christopher Swift, a Georgetown University professor and former Treasury Department official, told the House Armed Services Committee.

Beyond its implications for broader U.S. counterterrorism operations, the expulsion of ISIS is being examined by analysts at the CIA and the National Counterterrorism Center for clues to al-Qaeda’s evolving strategy amid the turmoil of the Middle East.

A U.S. intelligence official asked: “Does it make al-Qaeda leadership look stronger or weaker” to eject an insubordinate but violently effective franchise? “Will it help their recruitment efforts” to side with Jabhat al-Nusra, a faction in Syria that has stronger ties to that nation’s populace and is seen as marginally less extreme?

U.S. officials said the split is unlikely to harm the operational capabilities of ISIS, which has never depended on its al-Qaeda sponsor for funding or other material support. The officials, noting long-standing frictions between al-Qaeda leader Ayman al-Zawahiri and ISIS’s upper ranks, said there is no doubt the ban was real and not a ruse.

Current and former officials noted that Zawahiri did not cite ISIS’s brutality in the decision to sever ties with the group but instead focused on its battles with Jabhat al-Nusra and insubordination. Officials also said the move underscores the importance that Zawahiri attaches to Syria — over maintaining ties with a wayward but viable franchise in Iraq.

“Syria is an opportunity for them” above all others, said a former senior U.S. intelligence official. “They truly believe that if they don’t get in there and do the best they can, they’re nothing anymore.”