WASHINGTON >> The Pentagon has transferred a Guantánamo Bay prisoner to the custody of Saudi Arabia, a spokeswoman announced today. The handoff is the first time a detainee has left the wartime prison under President Donald Trump, who vowed to fill it back up but has instead overseen a reduction in its population.

The prisoner, Ahmed Muhammed Haza al-Darbi, is unlikely to be set free soon. U.S. officials intend for him to serve the roughly nine years remaining in a 13-year sentence he received after pleading guilty before a military commission to terrorism-related offenses involving a 2002 Qaida attack on a French-flagged oil tanker off Yemen’s coast.

“My words will not do justice to what I lived through in these years and to the men I leave behind in prison,” Darbi said in a statement provided through his volunteer lawyer, Ramzi Kassem, a law professor at the City University of New York. “No one should remain at Guantánamo without a trial. There is no justice in that.”

Darbi’s departure leaves 40 detainees at Guantánamo, down from 41 when President Barack Obama left office. The transfer comes as the Trump administration has been struggling to fulfill the president’s strong desire to back up his chest-thumping campaign rhetoric about Guantánamo, even as counterterrorism and security professionals, including Defense Secretary Jim Mattis, have repeatedly argued that other approaches made more practical sense.

Darbi was one of about 780 men whom the Bush administration took to the U.S. naval base in Cuba during the early chaotic years of the war on terrorism. President George W. Bush’s claims that he could hold people there indefinitely without trial or judicial review and without obeying the Geneva Conventions — as well as early photos of men in orange jumpsuits kneeling before the cages of Camp X-Ray, the hastily constructed initial phase of the prison — turned Guantánamo into a charged symbol around the world of prisoner abuses and U.S. power.

But over time, court interventions gave prisoners rights to hearings and to humane treatment under the Geneva Conventions, and conditions improved at the prison. In his second term, Bush started trying to close it because, as he wrote in his memoir, “the detention facility had become a propaganda tool for our enemies and a distraction for our allies.”

Obama, who inherited 242 detainees from Bush, shared that view and significantly winnowed the inmate population without adding new detainees. But Congress blocked him from bringing several dozen detainees deemed untransferable to a different prison on domestic soil.

By contrast, Trump boasted during the campaign that he would not only keep the prison open but also “load it up with some bad dudes.” But defying the expectations created by that rhetoric, his administration, too, has brought no new captives to Guantánamo — and now the population has instead shrunk.

A WHITE HOUSE PUSH

In January, Trump signed an executive order directing Mattis to recommend within 90 days a policy about how to handle future detainees, including whether or when to take them to Guantánamo. Announcing that order in his State of the Union address, Trump also ad-libbed a line that “in many cases,” future detainees will be sent to the prison.

The Pentagon said today that Mattis had provided updated policy guidance about when to propose transferring detainees to Guantánamo “should that person present a continuing, significant threat to the security of the United States.”

The Defense Department gave few details about the document. But people familiar with it said it was several pages long and consisted of screening criteria about what could make a terrorism suspect eligible for Guantánamo detention, without specifying when that option should be preferred over alternative dispositions. One person portrayed the document as vague, and another said it made no major changes from existing policy.

In recent years, the government has tried to leave lower-level detainees in the hands of allies, while interrogating important captives at an overseas military base or on a naval ship. After the questioning, U.S. officials have preferred to have an ally take or maintain custody of a suspect, with a fallback option of prosecution in civilian court.

Transfer to Guantánamo has been a theoretical last resort; no new captive has arrived there since 2008, when the Supreme Court ruled that prisoners there have a constitutional right to file habeas corpus lawsuits challenging the basis for their detention.

When Obama pursued that approach, it was repeatedly criticized by Republicans as weak. But those voices have largely grown quiet as it continued under Trump, although Sen. Lindsey Graham, R-S.C., has continued to argue that the United States needs a place where it can interrogate terrorism suspects without defense lawyers for a much longer period.

But many national security professionals see taking new detainees to Guantánamo as unattractive for several reasons. It is extremely expensive. Moreover, in practice, the combination of an interrogation followed by a civilian-court prosecution has successfully garnered critical intelligence while also resulting in convictions and harsh sentences.

By contrast, the military commissions system has struggled to get contested cases to trial, and a statute bars authorities from bringing detainees onto domestic soil, even for prosecution, once they have reached Guantánamo.

Finally, most of the terrorism suspects captured lately by the United States or allied forces have been associated with the Islamic State, like two British men who were recently caught in Syria by a Kurdish militia.

The Obama and Trump administrations have both contended that the legal authority Congress granted to the executive branch to use military force — such as detaining people without trial — against al-Qaida and for the Iraq War legitimately extends to the Islamic State. But it is not clear that their stance is lawful. Taking Islamic State detainees to Guantánamo would give a court an opportunity to rule that the larger conflict in Iraq and Syria is illegal.

A COOPERATIVE DETAINEE

During his time in U.S. custody, Darbi cooperated with investigators and lived apart from the main detainee population. A court document jointly prepared by prosecutors and defense lawyers for his sentencing said that his testimony against two other detainees facing tribunal charges was “unprecedented in similar counterterrorism prosecutions to date.”

Darbi had been detained for about 12 years before his 2014 guilty plea but received no credit for that time. Under the terms of the plea deal, Darbi was supposed to be transferred by February to Saudi Arabia, which runs a custodial rehabilitation program for Islamic extremists and where his family lives.

The question of whether Trump administration would renege on the Obama-era agreement had been closely watched, and it missed the deadline by several months — although Darbi blamed the Saudi government for dragging its feet. While belated, the transfer is a rare success for the commissions system at Guantánamo.

Kassem has represented several other current and former Guantánamo detainees, including Awad Khalifa, a Libyan man who was resettled in Senegal in 2016 but forcibly deported last month back to Libya, where he has apparently been imprisoned by a militia in Tripoli. He had expressed fear that he would be killed if repatriated.

Kassem also asked a judge last month to order an independent psychological examination of another client, Mohammed al-Qahtani. Qahtani, a Saudi, is believed to have been the intended 20th hijacker in the Sept. 11 attacks but was denied entry into the United States in August 2001 by a suspicious customs agent at the Orlando, Florida, airport.

Qahtani was tortured at Guantánamo during a more than 50-day interrogation in late 2002 and early 2003. Saying that Qahtani has a history of significant mental illness dating to a brain injury he suffered as a child, Kassem wants a court to order the military to send him back to the custody of Saudi Arabia, too.

In a statement, Kassem argued that the legal and security arguments for closing the prison were “overwhelming” and blamed politics for why it has remained open under three presidents.

“This is the first prisoner transfer under Trump, but it may also be the last unless the courts meaningfully check the president’s claimed power to imprison men without charge for as long as he pleases,” Kassem said.

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