President Barack Obama waves goodbye as he boards Air Force One for departure from RAF Station Fairford in Gloucestershire, England, en route to Joint Base Andrews, Md., Sept. 5, 2014.

President Barack Obama waves goodbye as he boards Air Force One for departure from RAF Station Fairford in Gloucestershire, England, en route to Joint Base Andrews, Md., Sept. 5, 2014.

President Barack Obama greets a family after his visit to Stonehenge in Wiltshire, England, Sept. 5, 2014.

President Barack Obama greets a family after his visit to Stonehenge in Wiltshire, England, Sept. 5, 2014.

President Barack Obama visits Stonehenge in Wiltshire, England, Sept. 5, 2014.

President Barack Obama visits Stonehenge in Wiltshire, England, Sept. 5, 2014.

President Barack Obama talks with National Security Advisor Susan E. Rice aboard Marine One en route to Stonehenge in Wiltshire, England following the NATO Summit in Wales, Sept. 5, 2014.

President Barack Obama talks with National Security Advisor Susan E. Rice aboard Marine One en route to Stonehenge in Wiltshire, England following the NATO Summit in Wales, Sept. 5, 2014.

President Barack Obama travels aboard Marine One en route to Stonehenge in Wiltshire, England following the NATO Summit in Wales, Sept. 5, 2014.

President Barack Obama travels aboard Marine One en route to Stonehenge in Wiltshire, England following the NATO Summit in Wales, Sept. 5, 2014.

First Lady Michelle Obama hugs LeBron James following the USA vs. France men’s basketball game at Olympic Park during the 2012 Olympic Games in London, England, July 29, 2012.

First Lady Michelle Obama hugs LeBron James following the USA vs. France men's basketball game at Olympic Park during the 2012 Olympic Games in London, England, July 29, 2012.

First Lady Michelle Obama watches the swimming finals and medal ceremonies at the Olympic Park Aquatics Center during the 2012 Summer Olympic Games in London, England, July 28, 2012.

First Lady Michelle Obama watches the swimming finals and medal ceremonies at the Olympic Park Aquatics Center during the 2012 Summer Olympic Games in London, England, July 28, 2012.

First Lady Michelle Obama greets former Olympic swimmer Summer Sanders, right, and her children, Skye and Spider, following a "Let’s Move! London" event at Winfield House in London, England, July 27, 2012. Sanders is serving as part of the U.S. delegation to the 2012 Olympic Games in London.

First Lady Michelle Obama greets former Olympic swimmer Summer Sanders, right, and her children, Skye and Spider, following a "Let's Move! London" event at Winfield House in London, England, July 27, 2012. Sanders is serving as part of the U.S. delegation to the 2012 Olympic Games in London.

First Lady Michelle Obama participates in a tug of war competition during a "Let’s Move! London" event at Winfield House in London, England, July 27, 2012.

First Lady Michelle Obama participates in a tug of war competition during a "Let's Move! London" event at Winfield House in London, England, July 27, 2012.

First Lady Michelle Obama talks with Joshua Wilkins-Waldron during a "Let’s Move! London" event at Winfield House in London, England, July 27, 2012.

First Lady Michelle Obama talks with Joshua Wilkins-Waldron during a "Let's Move! London" event at Winfield House in London, England, July 27, 2012.