U.S. forces drive armored vehicles near the village of Yalanli, on the western outskirts of the northern Syrian city of Manbij. (Delil Souleiman/AFP/Getty Images)

Since the summer of 2014, American forces have been fighting the so-called Islamic State (sometimes called ISIS) in Iraq — and, at least since the autumn of 2015, in Syria as well. Given the brutal nature of the ISIS terrorists, few objected to the airstrikes, front-line advisers, and then Special Operations personnel utilized against them.

But it has never been clear that this expanding use of force was legally authorized. House deputy whip Tom Cole (R-Okla.) recently wrote, “In the opinion of many on both sides of the aisle, President Obama conducted unauthorized and therefore illegal wars in Libya, Syria and other parts of the Middle East.”

The War Powers Resolution, for example, gives the president the power to use force for an extended period only when there is (1) a declaration of war; (2) a specific statutory authorization; or (3) “a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.”

The Obama administration test-drove several justifications for its attacks on the Islamic State, but wound up claiming Option (2): that these operations had specific statutory authorization. The catch was, it had come more than a decade ago, when Congress passed the Authorization for the Use of Military Force (AUMF) in response to the Sept. 11, 2001, terrorist attacks. The idea was that the Islamic State had once been associated with al-Qaeda and therefore fell under the 2001 AUMF’s umbrella.

The Islamic State did not exist in 2001, and the Obama team’s legal claims did not get great external reviews. Still, in the venue that counted — Congress — the rationale got a passing grade. If many believed the wars were unauthorized, as Cole claims, very few acted on that belief. “I believe the administration has the authorities to do what they’re doing against ISIS,” said the chair of the Senate Foreign Relations Committee, Bob Corker (R-Tenn.), in November 2015.

Indeed, when the administration sent legislators a new draft of AUMF specific to the Islamic State (though without repealing the 2001 version), it received no consideration in either chamber of Congress.

But the matter is back in the news — and that legislative inertia may be shifting. Importantly, last week Defense Secretary Jim Mattis testified in favor of a new AUMF. He didn’t think it was legally necessary, he told a Senate subcommittee, but said, “I think it’d be a statement of the American people’s resolve. I thought the same thing for the last several years, I might add, and have not understood why the Congress hasn’t come forward with this, at least to debate.”

And recently lawmakers, including Cole, Sen. Todd C. Young (R-Ind.), and Rep. Jim Banks (R-Ind.) have introduced legislation to do just that. Some versions would repeal the 2001 AUMF, others would not. H.J. Res 89, to take one example, provides open-ended authorization for the use of force “against al-Qaeda, the Taliban, the Islamic State of Iraq and Syria, successor organizations, and associated forces.”

Granted, this is a very broad delegation of power, without geographic or temporal limits. Even so — why introduce it now?

I’d like to believe these lawmakers have been perusing the very persuasive back issues of the Monkey Cage — not just those linked above, but going back to the Libya intervention of 2011. (They might add the new edited volume “The Imperial Presidency and the Constitution” to their reading list.)

But there are new facts — and new troops — on the ground. Soldiers from the 82nd Airborne Division are in Mosul, Iraq. Hundreds of — perhaps as many as a thousand or more — additional personnel are headed to Syria. Other hostile groups (such as al-Shabab in Somalia) have been thrust underneath the 2001 AUMF’s umbrella. The geographic regions designated as “zones of active hostilities” are being enlarged, removing extant constraints on airstrikes, raids and drones in those areas; changes in the rules of engagement may be linked to increased civilian casualties. And limits on the CIA’s paramilitary role, notably in conducting drone operations, have reportedly also been lifted.

Just as important an impetus, perhaps, is the fear that without an AUMF, the matter might transcend simple “resolve” — and wind up in court. The judiciary has long avoided taking a stand on the meaning of the War Powers Resolution, partly because Congress as a whole has never pushed the point. Last year, an Army captain took another route, bringing suit to argue that the president’s orders deploying him to Kuwait were unlawful because the Islamic State operation had not been legally authorized. A district court dismissed that case as well, holding the officer lacked standing to sue and that it was a “political question” the court could not decide.

But the Trump administration has made clear that it wants to expand the use of the Guantanamo Bay, Cuba, detention facility; Attorney General Jeff Sessions told USA Today that Guantanamo is “a very fine place for holding these kinds of dangerous criminals.” An executive order drafted for President Trump and unearthed by the New York Times’ intrepid Charlie Savage would revoke former president Barack Obama’s 2009 executive order closing the facility, and direct the military to hold Islamic State members there.

The first part of that would be a formality; Obama’s order set a deadline of January 2010, which proved wishful thinking given legislative opposition to housing the remaining detainees anywhere else. (About 45 are still in custody, of the 780 once housed there; over time most were transferred to other countries, more than 500 by the Bush administration and the remainder by the Obama administration.)

The second part, though, opens the courtroom door. The Supreme Court made clear in a series of cases starting in 2002 that those held at Guantanamo have certain due process rights. As Savage wrote in January, “Any Islamic State detainee would have legal standing to get a court to rule on the question of whether the group is legitimately part of the war against Al Qaeda.” Notably, the AUMF draft noted above (H.J. Res 89) specifically authorizes detention of “persons who are a part of or substantially supported al-Qaeda, the Taliban, the Islamic State of Iraq and Syria, any successor organization, or any associated force of those organizations” — in fact, detention “without trial until the end of the hostilities authorized by” the resolution.

A legal fight over the meaning of the War Powers Resolution — and perhaps a showdown over the release of detainees — is not one the administration wants. Presidents prefer the permissive precedents offered by the current history of congressional acquiescence to their use of force. And Congress, too, has found avoiding blame to be more agreeable than grappling with hard questions of war and peace. A new AUMF debate, we can hope, will fill in at least a few lines of the president’s current blank check.