A statement from a senior Administration official:

The 2001 AUMF authorizes the use of “all necessary and appropriate force against those nations, organizations, or persons” responsible for 9/11 and those who “harbored such organizations or persons.”  The Administration has interpreted the 2001 AUMF to authorize the use of force against AQ, the Taliban, and associated forces.  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.

Further remarks from the President’s speech this evening:

My Administration has also secured bipartisan support for this approach here at home. I have the authority to address the threat from ISIL. But I believe we are strongest as a nation when the President and Congress work together. So I welcome congressional support for this effort in order to show the world that Americans are united in confronting this danger.

Further thoughts on this to follow.