In his speech on September 10, President Obama laid out a new strategy for dealing with the terrorist group ISIS (also known as ISIL) in Iraq and Syria. The most notable element of the newly announced plan is expanded military action: The US will conduct a "systematic campaign" of airstrikes against ISIS in both countries.

One problem: The president doesn't have unlimited power to wage war as he pleases. He needs to have authority from the Constitution, Congress, or both in order for military action to be legal. The Obama administration claims that its actions against ISIS are within the bounds of the law — but the rationale is shaky at best.

On a press call prior to Obama's speech, a senior administration official said that the president has "constitutional and statutory authority to deal with the threat posed by ISIL" and that the administration believes that the 2001 Authorization for the Use of Military Force, or AUMF, gives the president the authority to conduct military operations against ISIS without further congressional approval.

In an email to the Guardian's Spencer Ackerman, a different official expanded on the administration's reasoning:

"Based on ISIL’s longstanding relationship with al-Qa’ida (AQ) and Usama bin Laden; its long history of conducting, and continued desire to conduct, attacks against U.S. persons and interests, the extensive history of U.S. combat operations against Isil dating back to the time the group first affiliated with AQ in 2004; and Isil’s position - supported by some individual members and factions of AQ-aligned groups - that it is the true inheritor of Usama bin Laden’s legacy, the President may rely on the 2001 AUMF as statutory authority for the use of force against Isil, notwithstanding the recent public split between AQ’s senior leadership and Isil.

In the 2001 AUMF, Congress authorized then-President George W. Bush 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." The law made no mention of ISIS.

What does the 2001 AUMF have to do with ISIS in 2014?

At the time of the 9/11 attacks, ISIS, (which was then called Jama'at al-Tawhid wal-Jihad), had no affiliation with al-Qaeda. In 2004 the group swore allegiance to al-Qaeda and became known as al-Qaeda in Iraq. However, they have since split again. In February 2014, al-Qaeda announced that it had severed ties with ISIS.

Even if the AUMF's exclusively past-tense language ("planned," "authorized," "committed," "aided," "harbored") can be interpreted to include al-Qaeda's future collaborators as well as its past ones, the administration is going even further here and interpreting the AUMF as a granting the president unlimited authority to use force against a group that is no longer assisting al-Qaeda and is now its rival.

Essentially, the administration is arguing that the 2001 AUMF authorizes the president to go to war against any organization that ever temporarily worked with al-Qaeda, even if the group in question didn't actually exist in 2001, and even if any subsequent collaboration has turned to enmity by the time the US takes military action.

Legal scholars have been quick to condemn the administration's reasoning. Writing in Opinio Juris, Cornell University Law School professor Jens David Ohlin concluded flatly that "the 9/11 AUMF does not cover ISIS," noting that, although ISIS is arguably the "gravest Jihadist threat to the peaceful world — a position once held by Osama Bin Laden," that does not mean that ISIS fits into one of the AUMF's categories for allowable targets.

Cardozo School of Law's Deborah Pearlstein, also writing in Opinio Juris, agreed that the AUMF "does not plausibly extend to ISIL." Harvard University's Jack Goldsmith, a former Bush administration lawyer, wrote in Time that the argument's "premise is unconvincing."

This argument is a stretch, even according to the administration's own broad view of the AUMF

The Obama administration has long argued that the AUMF extends beyond al-Qaeda and the Taliban to also cover "associated groups," but including ISIS within that category seems to be a dramatic expansion of even the administration's own definition.

Former State Department Legal Advisor Harold Koh explained the administration's theory of "associated groups" in a May 2013 speech at Oxford University. He said an "associated force" is an "organized, armed group" that has "actually entered the fight alongside al-Qaeda" against the US and is therefore "a co-belligerent with al-Qaeda in its hostilities against America."

Pearlstein pointed out that ISIS doesn't seem to meet those criteria. "Is ISIL organized? Surely," she wrote. "Has it 'entered the fight alongside al-Qaeda'? Absolutely not. Al-Qaeda and ISIL are fighting each other." American University's Jennifer Daskal, writing in Just Security, likewise took issue with the inclusion of ISIS within the "associated forces" category, declaring it "to say it mildly, a stretch."

This same argument could be used to justify war in Indonesia and Nigeria

The authority that the Obama administration is claiming here is incredibly expansive. As Goldsmith wrote, the administration is basically arguing that "Congress has authorized the president to use force endlessly against practically any ambitious jihadist terrorist group that fights against the United States."

Al-Qaeda works with groups all over the world — the UN's sanctions authority lists 67 organizations as current al-Qaeda affiliates. So if the 2001 AUMF authorized war against ISIS because of its temporary collaboration with al-Qaeda, even though the groups are no longer affiliated, then it would also authorize the president to go to war against, for instance, Boko Haram in Nigeria or Jamaah Islamiya in Indonesia, at any time in the future. It wouldn't matter if they are collaborating with al-Qaeda at the time of a military strike, because those groups were al-Qaeda affiliates at some point. It's extremely difficult to accept that Congress intended to authorize wars of unlimited duration in West Africa or Southeast Asia when it passed the 2001 AUMF.

For his part, Obama seems to have long embraced an expansive view of the AUMF — the difference is that he used to be a lot more troubled by it. In May 2013 he advocated its repeal, warning that "unless we discipline our thinking, our definitions, our actions, we may be drawn into more wars we don't need to fight.