President Obama’s new war against ISIS is off to a shaky legal start.

Obama’s using the law that authorized attacks against al Qaeda to justify his new fight in Syria and Iraq. One small problem: ISIS and al Qaeda are at each others’ throats. Legal experts were shocked to learn Wednesday that the Obama administration wants to rely on that 2001 Authorization for Use of Military Force against al Qaeda for the new ISIS war.

“On its face this is an implausible argument because the 2001 AUMF requires a nexus to al Qaeda or associated forces of al Qaeda fighting the United States,” said Robert Chesney, a professor at the University of Texas School of Law. “Since ISIS broke up with al Qaeda it’s hard to make that argument.”

Ideally, when beginning a new war like Obama is doing now, the president would ask Congress to declare it. That is the approach that he took a year ago when Obama came close to authorizing airstrikes against the Syrian government as a response to its chemical weapons attacks on rebel positions outside of Damascus.

This approach is also consistent with how Obama himself campaigned for the presidency in 2008. During the campaign he told The Boston Globe, “The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.”

But the presidency can change a man. At first Obama asserted his authority to unilaterally authorize airstrikes in Iraq because as the commander-in-chief of the U.S. military, he had a responsibility to protect U.S. citizens and facilities.

But almost as soon as he began the air campaign, Obama’s war got much broader. By the end of August, U.S. aircraft were striking ISIS targets in Iraq near the Mosul dam to protect Iraq’s critical infrastructure. In a few short weeks, the initial narrow aims of the war had expanded.

Now Obama seeks to expand that war even more. Addressing the nation Wednesday evening he said he reserved the right to strike ISIS targets in Syria. “I have made it clear that we will hunt down terrorists who threaten our country, wherever they are,” he said.

So this might be a good time then to ask Congress for the authority to actually conduct such a campaign. Not so fast. “I have the authority to address the threat of ISIL,” Obama said, using the administration’s preferred acronym for ISIS. And while Obama asked Congress to authorize new military training for the Syrian opposition and said it would be nice if Congress authorized his new war, he also said that he does not need its approval or permission.

Obama’s top advisers are now saying that the 2001 congressional resolution that created the war against al Qaeda, known as an Authorization for Use of Military Force (AUMF), provides the president with the legal basis for his new war. “We believe he can rely on the 2001 AUMF for the airstrikes he is authorizating against ISIL,” a senior administration official told reporters Wednesday in a briefing on the president’s war speech.

That is ironic politically because just this spring Obama asked Congress to narrow the 2001 AUMF or even consider phasing it out.

Now Obama is asserting that this broad resolution, which has been invoked by Obama and his predecessor to justify attacking al Qaeda-linked groups all over the Islamic world, would also apply to a terrorist organization that is itself at war with al Qaeda.

The AUMF from 2001 is a declaration of war against al Qaeda and its associated forces. In 2013 ISIS split from al Qaeda and has even attacked al Qaeda’s official franchise in Syria known as al-Nusra Front. Al Qaeda’s leader, Ayman al-Zawahiri, after pleading with ISIS to leave Syria to al-Nusra, formally kicked the group out of al Qaeda this year.

“I think they are going to get more heat for this implausible interpretation of the 2001 AUMF than they realize,” said Jack Goldsmith, a Harvard Law School professor who served as assistant attorney general at the Justice Department’s Office of Legal Counsel in 2003 and 2004.

Benjamin Wittes, a senior fellow and research director in public law at the Brookings Institution, said the legal argument was a “very thin reed.”

“If they are relying on the 2001 AUMF for this, then what the president is saying is, essentially: This war, like all wars, must end; we can’t have endless wars; stop me before I sin again,” he added.

Wittes said that he took an expansive view of what would constitute associated forces for the 2001 AUMF. But he observed, “Surely associated forces doesn’t mean forces that are actively hostile and have publicly broken with and been repudiated by al Qaeda. Whatever ‘associated’ means, I don’t think it means that.”

One Obama administration official said the argument that the new war is legal under the 2001 AUMF stems from the fact that ISIS began as a franchise of al Qaeda. Initially ISIS was known as al Qaeda in Iraq and at one point its leader, Abu-Bakr al-Baghdadi, pledged allegiance to al Qaeda in what’s known as an oath of Bayat.

But that argument would essentially ignore the fact that Baghdadi today has publicly broken with al Qaeda and declared himself the Caliph of the Muslim world.

Obama’s legal issues are not confined to U.S. law either. The administration has not explained how the strikes accord with international law.

According to the U.N. Charter, “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state.” Exceptions are granted for self-defense, or when the U.N. Security Council specifically authorizes it. The Obama administration worked hard to secure a U.N. Security Council resolution before using American military force inside Libya in 2011.

Washington in the past has also used international humanitarian law to justify use of military force without U.N. endorsement—for example, when U.S. forces attacked Serbia during the Clinton administration. President Obama said Libya had to be attacked to avert a genocide in Benghazi.

In Iraq, Yemen, and Somalia, host governments invited U.S. forces in, as opposed to Syria. But President Obama is not seeking any U.N. Security Council permission for this fight. He is not seeking permission from the government in Damascus to conduct airstrikes, and he is not asserting that the strikes are needed for humanitarian purposes. A draft U.N. Security Council resolution circulated this week makes no mention of authorizing such a war.

The closest international legal justification Obama would have, according to Chesney, is to assert the U.N. Charter’s right to collective self-defense. In this particular case, Chesney said, the right to self-defense is stronger for Iraq, which is being attacked by ISIS forces based in Syria.

Even Obama’s Democratic allies in Congress are queasy about the legal justifications used so far for this war, even as they endorse its aims. “It is my view that the president possesses existing authorities to strike ISIL in the short term, but that a prolonged military campaign will require a congressionally-approved Authorization for Use of Military Force,” Senate Foreign Relations Committee Chairman Bob Menendez said in a statement. “[We] will begin drafting a tailored AUMF to provide the president with the additional authority required by the War Powers Resolution to continue operations to thwart the ISIL threat.”

— with additional reporting by Josh Rogin