When President Obama vetoed the 2016 defense authorization bill five weeks ago — in part because it wouldn’t let him close the military prison at Guantanamo — he did it with broad publicity and a photo op. Last week, he quietly signed the $607 billion bill, along with five others, just before the Thanksgiving holiday. But in his statement accompanying his signature, he signaled the fight’s not over yet.

“As I have said repeatedly, the executive branch must have the flexibility, with regard to the detainees who remain at Guantanamo, to determine when and where to prosecute them, based on the facts and circumstances of each case and our national security interests, and when and where to transfer them consistent with our national security and our humane treatment policy,” he wrote. “Under certain circumstances, the provisions in this bill concerning detainee transfers would violate constitutional separation of powers principles.”

Hinting that he might use executive action rather than legislation to whittle the prison’s population, Obama cited four sections in the bill that aim to hinder the transfer of detainees, including the 48 who have been cleared to be sent to another country.

One reimposes and stiffens a requirement that the Defense Secretary certify that a transfer is in the national security interests of the U.S., as well as that the receiving country is not a state sponsor of terrorism, has sufficiently mitigated any risks, agrees to share information with the U.S., and is not under any threat, among others. Another bans transfers to Libya, Somalia, Syria and Yemen — and that could interfere with a writ of habeas corpus, Obama said. (The administration, however, is not currently contemplating these conflict-ridden countries as potential hosts because they cannot meet security requirements, and has not for some time.) Lastly but perhaps most importantly, he cited two sections that block the transfer of detainees to the United States or spending money to build or renovate prisons to hold them.

“In the event that the restrictions on the transfer of detainees in sections 1031, 1033, and 1034 operate in a manner that violates these constitutional principles,” Obama said, naming these areas of the law, “my administration will implement them in a manner that avoids the constitutional conflict.”

Related: Beyond Guantanamo

For several years, Obama administration officials have been building their case for executive action to close Guantanamo. They argue that congressional restrictions on the prison trammel the president’s authority to protect the U.S. under Article II of the Constitution. Just last month, spokesman Josh Earnest said, “I wouldn’t rule out the president using every element of his authority to make progress.”

But such threats have lawmakers digging in their heels.

“The president would be exceeding his authority,” new House Speaker Paul Ryan said last month, vowing that House Republicans would use “every means at our disposal” to block him — including, potentially, taking him to court.

But there is little precedent for such a lawsuit, and the Supreme Court is reticent to intervene in disputes between branches. Even Democratic lawmakers acknowledge they would have no real recourse.

“Congress has done their job. Whether you agree or disagree, they’ve spoken on Guantanamo in the bill,” said Senate Minority Leader Harry Reid, D-Nev., last month. “And so the president will do what he has to do. He’s going to sign the bill and then he’ll determine whether or not he has any administrative authority to do anything different than what’s in that bill.”

Obama might, for example, issue an “at the buzzer” executive order in January 2017.

But can he really do it as he’s walking out the door? I don’t think so,” said Harvard law professor Jack Goldsmith, who served in the Bush administration as assistant attorney general in the Office of Legal Counsel from 2003 to 2004, and before that, special counsel to the Defense Department.

Goldsmith told Defense One the administration would have to start preparations in “early 2016, if not sooner…You can’t just put people on an airplane on Jan. 18.”

For one thing: as the White House plan for closing Guantanamo is expected to note and administration officials confirm, all of the U.S. facilities that might accept detainees would need modifications, and using unappropriated funds to do so could itself be illegal.

But executive action could set an untoward precedent. Tellingly, both the White House and Pentagon decline to confirm whether legal counsel has deemed such a step within Obama’s authority, or even whether the question recently has been, is, or will be considered among senior officials.

In the meantime, former officials such as Goldsmith have begun to weigh in. Cliff Sloan, the former special envoy for Guantanamo closure at the State Department, and Gregory B. Craig, White House counsel in 2009, recently wrote an op-ed, “The president doesn’t need Congress’s permission to close Guantanamo.”

“It intrudes on a core function and prerogative of the commander in chief,” Sloan told Defense One. “The whole basis for holding the Guantanamo detainees is that they are law of war detainees and it’s hard to think of something more intrusive than telling the president the specific location where he must hold specific detainees, especially in the context where his considered judgement is that that specific location hurts national security and is far too costly.”

But Goldsmith argues that the legal challenges are large. “The first challenge is that he would need to make an aggressive constitutional argument to disregard the congressional transfer restrictions,” he said. “That’s a very hard argument to make. Congress clearly has authority to impose those restrictions, and it’s not clear on what convincing basis he would set them aside.”

The bigger problem may be Obama’s early pledges to behave differently than his predecessor, he said. “The arguments floated by former lawyers in the administration look a lot like arguments used early in the Bush administration to disregard torture restrictions.”

Sloan disagrees. “There is absolutely no equivalent between there,” he said, calling action on Guantanamo a narrow issue. “It certainly doesn’t speak to the very broad, expansive interpretation of presidential powers in other circumstances.”

Attorney General Loretta Lynch testified on Nov. 17 the Justice Department would follow the laws passed by Congress and signed by the president, and opponents seized on her comments as proof the question had been answered.

“With respect to individuals being transferred to the United States, the law currently does not allow for that, and that is not as far as I am aware of going to be contemplated given the legal prescriptions,” Lynch said.

But days later, when Obama did sign the defense bill into law, his signing statement signaled that the debate within the administration is still very much open. He has set up the argument that’s he’s exhausted all options and Congress has left him no choice — but will he use it?

“I’m an optimist,” said Sloan, on the president’s prospects for closing the prison for good. “I know there’s a lot of pessimism. It’s sort of well-founded, but I wouldn’t say it’s impossible.”

Goldsmith described it as more of a rock and a hard place. “If he doesn’t exercise the Article II override, he fails on this first-week promise to close Guantanamo, a legacy issue,” he said. “If he does exercise his Article II override, he goes out having failed in a very dramatic and unprecedented way on his pledges about Article II. I don’t think he can have it both ways. He’s going to have to choose.”

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