A sixteen-year old law authorizing the invasion of Afghanistan and pursuit of Osama bin Laden gives President Trump "sufficient legal authority" to protect the United States from terrorism, and a new authorization for use of military force (AUMF) is not needed, according to the State Department.

The department argued in a letter to the Senate that the fight against the Islamic State of Iraq and Syria is rooted in the 2001 law due to the dying caliphate's historical ties to al Qaeda. Any additional conflict with the Syrian government, the administration argued, derives justification from "the inherent right" to self-defense.

A bipartisan minority of lawmakers worry that the current fighting stretches the limits of the law, but the administration's letter put a damper on the idea of a congressional debate and vote on a new AUMF.

"The United States has sufficient legal authority to prosecute the campaign against al-Qai'da and associated forces, including against the Islamic State of Iraq and Syria (ISIS)," Charles Faulkner, who works in the State Department's bureau of legislative affairs, wrote to Senate Foreign Relations chairman Bob Corker. "Accordingly, the administration is not seeking revisions to the 2001 AUMF or additional authorizations to use force."

The letter was sent just before a closed Foreign Relations Committee hearing in which Secretary of State Rex Tillerson and Defense Secretary Jim Mattis were set to brief lawmakers on the current AUMF. The discussion follows a debate among House lawmakers on the AUMF, after GOP leaders stripped a repeal of the 2001 law out of a major annual defense bill.

"We are at war against an enemy that did not exist in a place we did not expect to fight, so how an AUMF that was passed 16 years ago, before I was in Congress, could possibly be stretched to cover this is just beyond belief to me," Rep. Tom Cole, R-Okla., said during a House appropriations meeting in June.

The administration disagreed, and noted that Iraq has requested immediate U.S. help in defeating ISIS in Syria. "Iraq has made clear that it faces serious threats of continued armed attacks from ISIS operating from safe havens in Syria; the Syrian government has shown it cannot or will not confront these safe havens," Faulkner wrote.

The State Department letter makes a bolder endorsement of the 2001 law than Mattis made during Senate testimony in March, when he said lawmakers should vote on a new AUMF.

"I think it'd be a statement of the American people's resolve," Mattis said. "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."

A majority of lawmakers agree with the administration that the 2001 law permits fighting against ISIS, which originated as a splinter group from al Qaeda. But there is disagreement among experts about whether the law is merely "adequate" or actually "sufficient," as the Trump team argues.

Much of the answer hinges on how broadly the administration is allowed to define the language authorizing war against al Qaeda and its affiliates. If the 2001 bill justifies the fight against ISIS, on the theory that the group is an outgrowth of al Qaeda in Iraq, that raises the question of whether it would also permit invasions of territories plagued by offshoots of ISIS.

"I don't know," retired U.S. Army Gen. Richard Gross, who served as legal counsel for the Joint Chiefs of Staff under former President Barack Obama, replied when asked that question during a recent House hearing. "You're starting to get more and more attenuated, and so the legal risk [increases] that you may say, ‘yes, this falls within the AUMF,' but reasonable minds can disagree and a court or someone else may say, ‘no, that wasn't what we intended with that law.'