Sen. Lindsey O. Graham (R-S.C.) told reporters that Mattis outlined the new rules of engagement during back-to-back briefings for Graham and Sen. John McCain (R-Ariz.), chairman of the panel. Graham added that he supported Mattis's plans, and that the secretary had pledged to work more closely with lawmakers to keep them informed about expanding operations and newly identified threats for Congress to exercise oversight authority.
"The war is morphing," Graham said. "You're going to see more actions in Africa, not less; you're going to see more aggression by the United States toward our enemies, not less; you're going to have decisions being made not in the White House but out in the field."
Graham said that other changes to the Pentagon's counterterrorism policy would include the adoption of a "status-based targeting" system for suspected terrorists, meaning troops will be able to use lethal force against a suspected member of a terrorist organization even if that person does not pose an immediate threat.
He said Mattis had informed them that the military would also be changing how it decides when to use ground troops and when troops should be deployed in more of an advisory role.
The changes come as lawmakers have pressed the White House and Pentagon for details about what led to the Oct. 4 ambush in Niger in which four U.S. Special Forces soldiers were killed. Graham indicated that lawmakers are determined to learn what led to the soldiers' deaths and whether they could have been prevented.
The ambush occurred near Niger's border with Mali, where al-Qaeda's North Africa branch has been battling both the government and a French-led coalition seeking to flush them from desert hideouts. Of specific concern is whether U.S. intelligence assets in the region failed to detect the existence of a threat to American personnel in the region.
Graham said it's too early to know for certain.
"In war you fail, you make mistakes and the whole goal is to learn from your mistakes and not repeat them," he said. "The one thing I don't want to do is jump to conclusions."
McCain threatened Thursday to use a subpoena if necessary to compel information from the Trump administration. Mattis appeared on Capitol Hill a day after McCain's warning. After meeting with McCain for about 15 minutes, Mattis told reporters that military investigators would provide him with information about the Niger attack "as soon as they can," but he would not commit to a specific timeline about when more information would be available.
As chairman of the Senate committee with primary oversight over the military, McCain argued, he must be better informed of such operations ahead of time.
But it remained unclear what kind of "operation" the slain soldiers were involved in. "The U.S. military does not have an active, direct combat mission in Niger," AFRICOM said in a statement Friday. It said about 800 military personnel there provide support to the U.S. Embassy in Niamey, the capital; and to support construction at what it called a "temporary, expeditionary contingency" being built at Agadez, in the middle of the country. The Washington Post reported in 2014 that the United States had received permission from the Niger government to construct a drone base near Agadez.
AFRICOM, the command said in the statement, provides "training and security assistance to the Nigerien Armed Forces, including support for intelligence, surveillance, and reconnaissance to facilitate their efforts to target violent extremist organizations." Security operations, it said, "are executed almost exclusively" by partnered security forces.
U.S. forces, Mattis said earlier this week, provide refueling, intelligence and surveillance support for more than 4,000 French troops. A permanent French air base is in Niamey. Trump, in a June letter to Congress listing overseas military operations under the War Powers Resolution, described the Niger deployments in similar terms, along with an additional 300 U.S. military personnel in neighboring Cameroon. Both missions were begun, and regularly reported to Congress, by the Obama administration.
The frustrations McCain expressed toward the Pentagon went further than just Niger. After several critical statements earlier in the week, however, he struck a more conciliatory tone after his Friday meeting with Mattis, noting that they were "clearing up a lot" and "continu[ing] to try to improve our lines of communication."
When asked, McCain said he would base his decision on subpoenaing the administration "on whether we get the witnesses or not."
Mattis pledged to be forthcoming, saying "when the Senate and the House calls, they always show up, is my policy."
"And I have the technology to make that happen," Mattis added.
Part of the Pentagon's new order, Graham said, is a pledge from Mattis that the military will brief lawmakers more regularly on the status of operations and new designations of threats as they are being made.
"I will insist, as the war expands and as the rules change to be more aggressive, that Congress is informed more often so that Congress can exercise our constitutional authority whether or not we want to authorize this operation through the appropriations process," Graham said.
But he warned against trying to "rein in" the military's new strategy through any type of congressional action, guessing that the new rules would only intensify a debate over an authorization for the use of military force (AUMF) against the Islamic State that is expected to begin in coming days.
Graham said that there would be a role for the intelligence community to play in particular in keeping Congress informed about the status of operations, through members of the intelligence committee. The chairman and ranking minority-party member of the Senate Armed Services Committee are ex officio members of the Senate Intelligence Committee.
Mattis and Secretary of State Rex Tillerson are due back on Capitol Hill on Oct. 30 for a hearing before the Senate Foreign Relations Committee about whether the administration believes a new authorization for use of military force is necessary.
According to Sen. Tim Kaine (D-Va.), co-author with Sen. Jeff Flake (R-Ariz.) of the Senate's most popular AUMF proposal, the Niger attack only accentuates the need for Congress to specifically authorize the military's operations against the Islamic State.
"The many questions surrounding the death of American service members in Niger show the urgent need to have a public discussion about the current extent of our military operations around the world," Kaine said in a statement. "A new AUMF is not only legally necessary, it would also send an important message of resolve to the American public and our troops that we stand behind them in their mission."
When asked, Mattis would not give his opinion on whether the Niger attack changed the debate about a new AUMF.
But Graham was adamant Friday that it should not.
"There'll be a lot of members of Congress who say, wait a minute, if you can go anywhere you want to go, you can kill anybody you want to kill, then we need to rein you in," Graham said. "That's not the way it works."
"It's up to Congress to have oversight of these operations and if we don't like what they're doing then we can cut off funding," Graham said.
He said that he expected McCain and Mattis to work out the frequency and conditions under which the Pentagon would brief Congress about ongoing operations under the expanded counterterrorism rules of engagement.
Related: Read more at PowerPost
Karen DeYoung contributed to this report.