U.S. President Barack Obama while signing an executive order closing the U.S. detention center in Guantánamo, Cuba, on January 22, 2009. REUTERS/Larry Downing On his second day in office President Barack Obama signed an executive order to close the Guantánamo Bay detention camp, established to indefinitely detain and interrogate suspected terrorists, within a year.

But more than four years later, 168 prisoners remain at the facility, including 86 who have been cleared for release.

Senior UN human rights experts are calling for the indefinite detention facility to be closed and its prisoners released as 100 prisoners participate in a hunger strike that has led to force-feeding.

So what happened to Obama's campaign promise and executive order?

In the new book "Dirty Wars: The World Is A Battlefield," investigative journalist and New York Times bestselling author Jeremy Scahill argues that the choice to keep the Gitmo was made around January 2010.

From "Dirty Wars" (emphasis ours):

In early 2010, the Obama administration canceled the scheduled repatriation of more than thirty Yemenis held at Guantánamo who had already been cleared for release.

"Given the unsettled situation [in Yemen], I've spoken to the attorney general and we've agreed that we will not be transferring additional detainees back to Yemen at this time," President Obama said on January 5.

Lawyers for some of the Yemeni prisoners called the decisions "unconscionable," saying it would "effectively prevent any meaningful progress towards closing Guantánamo, which President Obama has repeatedly argued will make our nation safer."

It was clear that for the Obama administration, the Gitmo issue, a central pillar of the president's election campaign, was far less pressing than its counterterrorism agenda in Yemen, which had more citizens at the prison than any other nation."

The seeds of change began with an al Qaeda resurgence in Yemen from 2007 to 2009, as Scahill details.

In October 2008 U.S. Special Operations Forces were already carrying out "unilateral, direct actions" against al Qaeda suspects in Yemen, special ops veterans told Scahill.

Within days of Obama signing the Gitmo pledge, a former detainee named Said Ali al Shihri appeared in a YouTube video with three others to announce the creation of al Qaeda in the Arabian Peninsula (AQAP).

The U.S. subsequently expanded military action in Yemen, and in the summer Obama authorized the expansion of special ops capture/kill missions in the country.

By December 2010 Joint Special Operations Command was operating its own drones in Yemen as ground and air raids increased further.

Today, 91 of Gitmo's 166 detainees are from Yemen while the country is the second biggest front (behind Pakistan) in America's drone war against suspected terrorists.

Last month President Obama renewed his vow to close Gitmo, saying that the facility is "expensive," "inefficient," damaging to America's international standing, and "a recruitment tool for extremists."

Nevertheless, if the president to make good on his promise, the first move may have to be some sort of counterterrorism breakthrough in Yemen.

Buy "Dirty Wars" by Jeremy Scahill >