So even when Obama, in 2014, announced the U.S. would target ISIS with airstrikes, he said the 2001 law gave him authority because the group had grown out of al-Qaida. Months later, he asked Congress to pass a new war powers resolution to address ISIS more specifically and replace the outdated law.

Congress never acted. Obama, unbowed, stuck by his legal argument that the 2001 version was still sufficient.