President Barack Obama disembarks Air Force One upon arrival at Brussels National Airport in Brussels, Belgium, June 4, 2014.

President Barack Obama disembarks Air Force One upon arrival at Brussels National Airport in Brussels, Belgium, June 4, 2014.

President Barack Obama talks with former President Bill Clinton before an event in McLean, Va., Sunday, April 29, 2012.

President Barack Obama talks with former President Bill Clinton before an event in McLean, Va., Sunday, April 29, 2012.

President Barack Obama meets with NCTC Director Michael Leiter, center right, leadership and analysts in the secure video teleconference room at the National Counterterrorism Center in McLean, Va, Oct. 6, 2009.

President Barack Obama meets with NCTC Director Michael Leiter, center right, leadership and analysts in the secure video teleconference room at the National Counterterrorism Center in McLean, Va, Oct. 6, 2009.