Abstract: Despite massive changes in the geographical scope of the conflict that began on 9/11, the strategy and tactics employed, and the identity of the enemy, the 2001 Authorization for Use of Military Force (“AUMF”) remains the principal legal foundation under U.S. domestic law for the President to use force against and detain members of terrorist organizations. For many years, President Obama proclaimed that he wanted to repeal the AUMF and end the AUMF-authorized conflict. By the closing year of his presidency, however, his administration had established the AUMF as the legal foundation for an indefinite conflict against Al Qaeda and associated groups and extended that foundation to cover a significant new conflict against the Islamic State. This transformation of the AUMF is one of the most remarkable legal developments in American public law in the still-young twenty-first century, and it will stand as one of President Obama’s primary legal legacies. In addition to establishing this descriptive claim, this Essay considers how the Obama administration has invoked international law in making arguments about the scope of the AUMF. As the Essay explains, although the Obama administration often maintained that international law was an important constraint on its actions, on a range of issues where international law was unsettled it interpreted it in ways that supported presidential discretion and flexibility under the AUMF.