The AUMF was designed to retaliate against those responsible for the 2001 attacks. But it has been stretched to permit lethal U.S. strikes against al-Qaida's many allied affiliates, including extremists and guerrilla groups that have shown little or no interest in attacking American targets.
The Obama administration has appeared reluctant to scale back those authorities, which lets it conduct drone strikes on suspected terrorists in North Africa and the Mideast.
"Make no mistake, our nation is still threatened by terrorists," Obama said last May in a speech in which he also repeated his demand that Congress allow trials and transfers for most Guantanamo detainees.
As it stands, the legal authority to hold detainees at Guantanamo will continue until either the president or Congress declares the fight over. Federal courts are gearing up to consider cases from Guantanamo detainees who, eyeing the looming end of the war in Afghanistan, will argue the law is no longer valid.
The chairman of the U.S. Senate Armed Services, Sen. Carl Levin, said it's unlikely that either Congress or the White House will let the 2001 law expire. "As long as there is an al-Qaida that is threatening the U.S., no one is probably going to try that," Levin, D-Mich., told The Associated Press.
Levin wants to allow some detainees to be transferred to other nations or trial in the U.S., and has included that in the 2014 Defense Department legislation that the Senate is considering after failing to approve it last week.
If any troops remain in Afghanistan even as trainers, as expected, then technically the U.S. still would be involved in active hostilities in Afghanistan, and "then at least arguably, the AUMF could still be in effect," Levin said.
For the first time in years, senior administration officials held a closed hearing of a periodic review board this past week to start reconsidering the cases of 46 detainees who earlier were deemed too dangerous to release.
Most are from Yemen, where lawmakers say al-Qaida is too strong to risk releasing a detainee who might be easily re-recruited to jihad. But many never will be tried in a U.S. court because the government is unwilling to reveal its evidence in their cases, probably because it was obtained during harsh interrogations or though other classified methods.
Obama acknowledged in his May speech that it was unclear what will happen to those detainees if he were to close Guantanamo. But he expressed confidence "that this legacy problem can be resolved, consistent with our commitment to the rule of law."
Six months later, administration officials say there's been little progress so far, and U.S. Sen. Saxby Chambliss of Georgia, the top Republican on the Senate Intelligence Committee, said in a statement to the AP that those detainees are "among some of the most dangerous terrorists in the world. They belong at Guantanamo."
He called Obama's plan to close the detention facility "irresponsible."
Pentagon lawyers have decided that an estimated 15 to 20 detainees can be tried in a military court. The cases of more than a dozen, including Sept. 11 mastermind Khalid Sheikh Mohammed, are already being prosecuted.
An additional 84 detainees have been cleared for transfer, but are waiting for the U.S. to release them to nations that either will take them or are deemed secure enough by Congress to accept them. More than 50 of them are Yemeni.
William Lietzau, who retired from the Pentagon in August after more than three years as the deputy assistant defense secretary overseeing detainee policy, said the continued detentions puts the U.S. at risk of slipping into a perpetual state of quasi-war that has a dubious legal basis. He said the government needs to decide when it is no longer at war to keep it from relying on legal authorities that should be used only in cases of last resort.