One sweltering afternoon last month, a Boeing C-17 military transport plane arrived at the American naval base here. It had come to take six low-level detainees to new lives in Uruguay after 12 years of imprisonment.
Days before, Vice President Joseph R. Biden Jr. had called Uruguay’s president, José Mujica, pressing him to resettle the men. The foreign leader had offered to accept the detainees last January, but by the time the United States was ready for the transfer this summer, Mr. Mujica was worried that it would be politically risky to follow through because of coming elections in his country, according to Obama administration officials.
After four days of frantic negotiations between the two governments as the plane sat on the tarmac, the C-17 flew away without its intended passengers.
Although President Obama pledged last year to revive his efforts to close Guantánamo, his administration has managed to free just one low-level prisoner this year, leaving 79 who are approved for transfer to other countries. It has also not persuaded Congress to lift its ban on moving the remaining 70 higher-level detainees to a prison inside the United States.