Pakistani Prime Minister Nawaz Sharif met with President Obama at the White House on Wednesday. Dennis Brack/pool/Getty Images hide caption

itoggle caption Dennis Brack/pool/Getty Images

An American flag flying over Camp VI, where detainees are housed at the U.S. Naval Base at Guantanamo Bay. Bob Strong /Reuters /Landov hide caption

itoggle caption Bob Strong /Reuters /Landov

An unmanned drone armed with Hellfire missiles is shown over southern Afghanistan. A Hellfire missile fired from a drone was used in 2011 to kill an American in Yemen who the Obama administration says was an al-Qaida leader. Another American died in that attack, and a 16-year-old American was killed in a separate drone strike. Lt. Col. Leslie Pratt/AP hide caption

itoggle caption Lt. Col. Leslie Pratt/AP

John Brennan, President Obama's nominee to head the CIA, prepares to testify at his confirmation hearing before the Senate Intelligence Committee on Thursday. J. Scott Applewhite/AP hide caption

itoggle caption J. Scott Applewhite/AP

October 2011: Men stand on the rubble of a building destroyed by a U.S. drone strike in southeastern Yemen. Among those killed was U.S. citizen Abdulrahman al-Awlaki, the son of U.S.-born cleric Anwar al-Awlaki — who himself was killed by a drone strike the month before. Khaled Abdullah /Reuters /Landov hide caption

itoggle caption Khaled Abdullah /Reuters /Landov

President Obama and John Brennan, his top counterterrorism adviser, in the Oval Office on Jan. 4, 2010. Brennan is a key voice about who gets put on the "kill list." Pete Souza/White House hide caption

itoggle caption Pete Souza/White House