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WASHINGTON, USA - MAY 27: NATO Secretary General Jens Stoltenberg speaks and answers questions on challenges facing the Transatlantic Alliance and the ways in which NATO must adapt at a Statesmen's Forum event at the Center for Strategic and International Studies (CSIS) in Washington, USA on May 27, 2015. (Photo by Samuel Corum/Anadolu Agency/Getty Images)

WASHINGTON, USA - MAY 27: NATO Secretary General Jens Stoltenberg speaks and answers questions on challenges facing the Transatlantic Alliance and the ways in which NATO must adapt at a Statesmen's Forum event at the Center for Strategic and International Studies (CSIS) in Washington, USA on May 27, 2015. (Photo by Samuel Corum/Anadolu Agency/Getty Images)