(CNN)The Pentagon on Monday announced that 15 detainees held at the US detention camp in Guantanamo Bay, Cuba, had been transferred to the United Arab Emirates -- the largest single transfer of detainees since President Barack Obama assumed office in 2009 after campaigning on a platform that included closing the controversial camp.
What to know about Guantanamo Bay
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Will Obama get to keep his promise? The number of detainees has decreased over the years, but the camp -- nicknamed Gitmo -- remains open due to continued opposition from Congress and a bureaucratic system that draws out the process for transferring prisoners.
Here's what you need to know:
The detention facility opened following the 9/11 terror attacks and was most active during President George W. Bush's administration.
It was intended to be a place where suspects in the war on terror could be interrogated. But prisoners have been indefinitely detained, many without charges or trial and subjected to reported abuse. As the war dragged on, Gitmo became an international symbol of US rights abuses in the post-9/11 era.
The detention camp is located at Naval Station Guantanamo Bay. The "Gitmo" designation comes from the airfield code, GTMO, and predates the detention facility.
There were 684 detainees in June 2003. Six years later, when Obama took office, there were 242 left.
The youngest detainee was just 14 years old when he was captured in Pakistan in 2002, according to his lawyer. One of the oldest, Mohammed Sadiq of Afghanistan, was 89 when he arrived at Gitmo in 2002. He was released after a few months in captivity.
Men from 49 nations have been detained at Gitmo. Initially, the prisoners were combatants captured during the US war in Afghanistan in 2002, but many of those prisoners were later identified as being low-level fighters or, in some cases, civilians. Most of the detainees (over 200) were citizens of Afghanistan, followed by Saudia Arabia and Yemen. Many of the remaining detainees are from Yemen.
The first detainees arrived in early 2002. Fourteen years later, some of them are still there.
Sixty-one detainees remain imprisoned at Gitmo. Twenty have been approved for release. Most of the others cannot be transferred abroad because they're considered too dangerous. Obama hopes to move them to detention facilities in the United States.
Among those imprisoned at Gitmo are Khalid Sheikh Mohammed, the so-called mastermind of the September 11th terror attacks, and Walid Muhammad Salih bin Attash, who admitted to helping bomb the US Embassy in Kenya in 1998 and the USS Cole in Yemen in 2000.
The nonprofit group Human Rights First says 59 nations have accepted detainees, including Afghanistan, Pakistan, Saudi Arabia, Yemen, the United Arab Emirates, Oman, Algeria, Britain, Morocco, Kuwait and Qatar.
Although most of the remaining detainees are Yemeni nationals, the Obama administration refuses to repatriate them due to their country's instability.
Closing Gitmo was a campaign promise. "Keeping this facility open is contrary to our values," Obama said earlier this year.
Criticism of Gitmo stems from numerous issues, including: the alleged torture of prisoners, the holding of men who weren't combatants or terrorists, the desecration of prisoners' Qurans by guards and the force-feeding of hunger-striking prisoners. Nine prisoners have died, with three committing suicide on the same day.
The legality of Gitmo has been questioned since its inception. The Bush administration argued the detainees should not receive constitutional protections because they aren't held on US soil -- an idea rejected by the Supreme Court in 2004. Since then, hearings for prisoners have been a stop-and-go process, with ongoing discussions about whether trials should be held on US soil.
Critics of the detention facility argue that its existence makes it difficult for the US to complain about brutality and human rights abuses in other countries.
Obama signed an executive order in early 2009 to shutter Gitmo within one year, but quickly admitted the US was having trouble finding other countries to accept prisoners.
Last February he unveiled a plan to transfer the remaining detainees to other countries or to sites in the United States. Options for housing prisoners in the US include the federal Supermax prison in Florence, Colorado; the military prison in Leavenworth, Kansas; and the Naval Consolidated Brig in Charleston, South Carolina.
Many Republicans leaders don't want Guantanamo Bay closed. They believe Gitmo is an important tool for the US in its fight against terrorists and are worried about the danger of putting such detainees in US prisons. A majority of Americans also oppose bringing detainees to US soil, according to a 2016 CNN/ORC poll.
What's more, Congressional Republicans say released detainees will return to terror. Of the 676 detainees released from the detention facility as of January, 118 have returned to the fight and a further 86 are suspected of returning, said a recent report from the Director of National Intelligence.
It's never been easy to find other countries that will accept large numbers of people believed to be terrorists, especially with the rise of ISIS. Lee Wolosky, the US Special Envoy for Guantanamo Closure, said: "Frequently we have little to offer them in return, except the continued goodwill of the United States."
Republicans say language in two bills Obama signed -- the defense authorization and defense appropriations bills -- bar transferring detainees to the US. Even members of the Obama administration say the law currently prohibits that.
In a letter to Congress, Army Lt. Gen. William Mayville said, "Current law prohibits the use of funds to 'transfer, release, or assist in the transfer or release' of detainees from Guantanamo Bay to or within the United States."
The White House might argue those transfer bans are unconstitutional and take the case to court, but it probably wouldn't be settled until after Obama's term in office ends.