U.S. President Barack Obama announced Tuesday he's making one last effort to close down the notorious Guantanamo Bay detention centre, calling it a stain on the U.S. record of upholding the rule of law.

The president has been attempting to close the military prison since the day he took his oath of office in early 2009.

"For Obama, it's an important promise he needs to fulfil," said Chris Edelson, an assistant professor at the School of Public Affairs at American University in Washington, D.C.

But there are significant legal and political challenges to closing it, and it's unclear where certain detainees would be housed instead.

Gitmo's history

The U.S. has held the 116-square-kilometre naval base on the eastern tip of Cuba since 1898. Since 1903, it has paid Cuba about $4,000 a year to keep its base at that site.

However, when the Cuban Revolution took place in 1959, Cuba stopped accepting those cheques and hasn't cashed one since.

Since 2002, the base has been used to house people that the U.S. says are involved in the "global war on terror." At its peak in 2005, Guantanamo housed more than 500 detainees from more than 40 countries.

'Forever prisoners'

Today, there are 91 men left at the detention centre.

Thirty-five have been cleared for release, 10 have been charged and are going through military commissions, and the remaining 46 have never been charged with a crime, but have been deemed too dangerous to release, according to Peter Jan Honigsberg, a law professor at the University of San Francisco who runs the organization Witness to Guantanamo.

"Those men have been called in some circles 'forever prisoners' because they can't be charged and they can't be transferred and released," said Honigsberg. "And so who knows what will happen to them."

He and Edelson said those men can't be charged for a lack of evidence, or because of tainted evidence.

Honigsberg said the Geneva Conventions require all prisoners to be released at the end of hostilities — but that specific end is open to interpretation.

"In the olden days, you had a clear start to war and end to war. That's not true now," he said. "These men are in limbo, caught betwixt and between."

Obama Guantanamo

On Tuesday, President Barack Obama announced the Pentagon's long-awaited plan to shut down the detention centre at Guantanamo Bay, Cuba, and transfer the remaining detainees to a facility in the U.S. (Pablo Martinez Monsivais/Associated Press)

Political gridlock

There are both legal and political challenges to closing the centre.

"What do you do with this category of people who the Obama administration says are too dangerous to release but can't be put on trial?" asked Edelson.

The administration started what are called "Periodic Review Boards" to check every few years whether it was still reasonable to hold these men without charges. Edelson said that's a good solution, but it's not used as much as it could be.

Another legal challenge is that every year, Obama has signed legislation stating he will not allow the detainees on U.S. soil. In order to change that, he has to get the law changed, which would require congressional support.

The Republican-dominated Congress is unlikely to give Obama that.

"If I'm a Republican in Congress, I'm going to say 'why would I give the president a political victory?'" said Edelson.

As commander-in-chief of the military, Obama could potentially use an executive order to move the detainees to U.S. soil.

"You can imagine all the hell that would break loose if he does that because Republicans will claim that he's moving beyond his constitutional powers," said Honigsberg. "I don't know that he has the guts to stand up to Congress and do that."

'Gitmo North'

On the political side, there are some who argue that the centre should remain open for national security reasons.

Both Edelson and Honigsberg agree that side of the debate has been much louder in the U.S. than the debate about closing the centre over human rights violations.

"It's unfortunate, but that side of the debate does not involve reason, it does not involve evidence. It involves emotion and fear," said Edelson. "The Obama administration has not gotten those points across very well."

If the president is successful in closing Guantanamo Bay, he still has to house those 46 men in limbo.

His suggestion of housing them at detention centres in the U.S. — provided Congress approves such a move — has come under fire as an empty gesture.

"His critics have called it 'Gitmo North' — they say it would just be a new prison in the United States that would continue to hold people indefinitely," said Edelson.

What's next?

Obama has less than a year to fulfil the promise he made in 2009.

"[Guantanamo] is a stain that is very clear on America," said Honigsberg. "The best thing to say would be: 'We made a mistake and we acknowledge that mistake and we take some accountability for that mistake and we will move on.'"

As it stands, neither Edelson nor Honigsberg think Obama has much of a chance of getting his way.