Senator Tim Kaine, in a statement on Monday, marked out the position that expanded military operations against ISIS are not covered by Congress’s 2001 authorization for use of military force (AUMF). Sen. Kaine then stated that he is “encouraged by reports that indicate Administration officials have signaled that seeking Congressional authorization for U.S. military action against ISIL is being considered” (my emphasis added). Reports indicate, however, that the administration is also still considering the opposite—that it does not have to go back to Congress on the ground that the 2001 AUMF applies to ISIS. The NSC spokeswoman recently told Defense One, “We are reviewing the applicability of the 2001 AUMF to this situation” (see also here).

I have previously written that the 2001 authorization does not cover ISIS, and I noted: “As readers of Just Security, Lawfare, and Opinion Juris know, a remarkable consensus of opinion has emerged across our blogs that ISIS is not covered by the 2001 AUMF.” One of the reasons is that ISIS split off from al-Zawahiri’s Al Qaeda central at least as far back as February 2014 (the fissure was evident before then).

So what explains this disconnect? What might be the administration’s theory for application of the 2001 AUMF to ISIS?

As Marty Lederman has explained, the administration has long maintained that the general legal framework for a group to be covered under the AUMF is two-fold: (1) the group has entered the fight alongside Al Qaeda and (2) the group is engaged as a co-belligerent in directing hostilities against the United States or its coalition partners.

[As an aside, last year I discussed an alternative ground in international law—in addition to the co-belligerency standard—as a basis for expanding the use of force to other parties to a conflict.]

We know that the administration takes the view that Al Qaeda in the Arabian Peninsula (AQAP) in Yemen meets the two-part standard. And thus the US is also battling AQAP as a party to the conflict. So, what if AQAP were to work in collaboration with another military organization in directing hostilities against the United States? Is that the legal hook that the administration is considering?

AQAP has been sharply divided over whether to support ISIS. That situation recently changed. In a statement on August 17, AQAP officially supported ISIS against the United States:

“We announce solidarity with our Muslim brothers in Iraq against the crusade. Their blood and injuries are ours and we will surely support them”;

“We assert to the Islamic Nation [all Muslims worldwide] that we stand by the side of our Muslim brothers in Iraq against the American and Iranian conspiracy and their agents of the apostate Gulf rulers.”

ISIS and AQAP may also be involved in training each other’s fighters, and AQAP has publicly provided ISIS advice on how to evade US airstrikes (see also here).

It is notable that one of the missing pieces is the direction of hostilities against the United States. That situation may, of course, itself change. In that respect, it is also relevant that the White House chose to describe the killing of journalist James Foley as “a terrorist attack against our country.”

I do not evaluate here the viability of this theory. It is important, as an initial matter, to know what the theory might be. Maybe a journalist should ask the administration a follow-up question.