The transfer of the man, Abdul Latif Nasser, 56, was the first sign of a renewed effort under President Biden to winnow the population of prisoners by sending them to other countries that promise to ensure the men remain under security measures. Mr. Nasser was never charged with a crime. The transfer process, which was pursued by the George W. Bush and Barack Obama administrations, had atrophied under Donald J. Trump. With Mr. Nasser’s departure, there are now 39 prisoners at Guantánamo, 11 of whom have been charged with war crimes. At its peak in the years after the Sept. 11, 2001, attacks and the invasion of Afghanistan, the prison complex at the U.S. naval base there held about 675 men. Far more complex policy decisions about transfers await the Biden team, including whether to transfer a mentally ill Saudi man, Mohammed al-Qahtani, who was tortured at Guantánamo and is considered to have been one of several candidates to be a potential 20th hijacker on 9/11.