Mattis and Tillerson discussed the prospects for a new war authorization behind closed doors with the Senate Foreign Relations Committee, where lawmakers said they had a productive discussion but aren't necessarily closer to resolving the prickly issues that have kept Congress from passing an Authorization for Use of Military Force (AUMF) since the US began fighting ISIS in 2014.
Sen. Tim Kaine, who has led the push in the Senate to vote on the ISIS war, said he was encouraged by the discussion with Mattis and Tillerson.
"They were very open to the idea of working on an authorization, not because they feel like they legally have to have it, but they think for the mission itself it would be good to have Congress engaged in that way," the Virginia Democrat told reporters after leaving the classified setting.
But the Trump administration made clear ahead of Wednesday's briefing that the military did have the legal authority it needed. The State Department sent Senate Foreign Relations Chairman Bob Corker a letter Wednesday stating the US "has sufficient legal authority to prosecute the campaign against al Qaeda and associated forces, including against the Islamic State."
"They were very clear in their responses -- the administration is not seeking an AUMF," Corker said. "I'm confident they'll work with us. They're not seeking one. But I think they saw there's an effort to do something."
Nevertheless, the closed briefing with national security leaders was another step signaling renewed momentum on Capitol Hill for Congress to finally vote on the war.
But there are still significant roadblocks to passing a new Authorization for Use of Military Force to replace the 2001 authorization still being used by the US military to fight ISIS, al Qaeda and others across the globe.
A small group of lawmakers from both parties have long been pushing for Congress to pass a new authorization for the war against ISIS, and their effort received a jolt in the arm in June when a House panel voted to repeal
the 2001 war authorization, a 60-word broad authorization approved just days after the Sept. 11, 2001 terror attacks.
The amendment to the defense appropriations bill from Rep. Barbara Lee -- a California Democrat who was the only member of Congress to vote against the 2001 AUMF -- would have repealed that authorization while giving Congress eight months to pass a new one.
But her effort was stripped from the underlying defense bill
by House Republican leadership when it went to the floor last week, as House Speaker Paul Ryan argued the spending measure was an inappropriate means to debate the war.
In the Senate, the Senate Foreign Relations Committee is holding hearings on the issue, but so far Senate leadership hasn't indicated a desire to pivot to the issue on the floor.
"Mitch McConnell has so many fires right now, that the last thing he wants to do is something that doesn't have a predictable outcome," a Democratic senator told CNN.
The problem with trying to pass a war authorization is twofold: Many lawmakers are still wary of the way that the 2002 vote for the Iraq War was used against Hillary Clinton in both the 2008 and 2016 presidential elections.
In addition, while most lawmakers support a new war authorization to fight ISIS, there are major disagreements over the details, including whether to allow or restrict US ground troops, the length of time for the authorization and whether to have geographical restrictions.
Senate Foreign Relations Chairman Bob Corker, whose committee is hearing from Mattis and Tillerson Wednesday afternoon, said he's still assessing what the next steps are to debate and mark up a war authorization, with a public hearing the next likely step.
Kaine and Republican Sen. Jeff Flake of Arizona introduced a war authorization
in May, which they hope is a bipartisan path to voting on the ISIS plan.
The Trump administration, like the Obama administration before it, believes it has the legal authority to fight ISIS with the 2001 war authorization, and it has not requested an updated version. The Trump administration also asserted it had legal authority to strike the Syrian government based on the same authorization.
Still, at a Senate hearing earlier this year, Mattis endorsed the idea of Congress voting to endorse the ISIS war. "I think it would be a statement of the American people's resolve if you did so," he said.
Corker and Maryland Sen. Ben Cardin, the top Democrat on the Foreign Relations panel, said they're working toward marking up a new war authorization. Corker said Wednesday that the discussion was helpful in beginning to address some of the sticking points on a new authorization.
"Secretary Mattis was very helpful in talking through some of the caveats that people might want to put in place," Corker said.
A key issue to watch will be how the bill would deal with the issue of ground troops -- many Republicans want no restrictions on the commander-in-chief and most Democrats argue they're hoping to prevent another open-ended US ground war in the Middle East.
"It can't be a lessening, in my opinion, of the options the president has," said North Carolina Republican Sen. Thom Tillis.
But Cardin told reporters Tuesday he believes the new war authorization should allow for US troops to conduct special operations and other targeted missions in the ISIS war, but that US ground forces should be prohibited.
He argued many Republicans are coming around, too, an idea that would be tested if a war authorization did come to the Senate floor.
"I think the landscape has changed in two years, I really do," Cardin said. "I think there's now more and more members of the Senate who recognize ground troops is not the answer."