Flags of member nations wave outside NATO headquarters in Brussels. For just the fifth time in its 66-year history, NATO ambassadors met in an emergency, Article 4 session to gauge the threat that the so-called Islamic State poses to Turkey. Geert Vanden Wijngaert/AP hide caption

itoggle caption Geert Vanden Wijngaert/AP

U.S. Bradley fighting vehicles take part in a joint exercise with Georgia at a military base near Tbilisi. Bradleys would be deployed to NATO member states in Eastern Europe and the Baltics if a Pentagon proposal is approved. Zurab Kurtsikidze/EPA/Landov hide caption

itoggle caption Zurab Kurtsikidze/EPA/Landov

Helicopter belonging to the Netherlands participates in NATO's Dynamic Mongoose anti-submarine exercise in the North Sea, off the coast of Norway, on May 4, 2015. MARIT HOMMEDAL/AFP/Getty Images hide caption

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Secretary of State John Kerry shakes hands with Ukrainian President Petro Poroshenko in Kiev Thursday. The U.S. and its NATO allies are discussing how to help Ukraine maintain its borders against a separatist movement. Jim Watson/AFP/Getty Images hide caption

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Supreme Allied Commander Gen. Philip Breedlove attends a media conference at NATO headquarters in Brussels on Jan. 22. Breedlove told NPR on Thursday that "the conditions in eastern Ukraine have to change." Geert Vanden Wijngaert/AP hide caption

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NATO has reported a spike in Russian military activity at sea and in NATO and other western European countries' airspace as tensions have mounted over Ukraine. Here, a Russian military long-range bomber aircraft flies in international airspace on Oct. 29. Royal Air Force/AP hide caption

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Estonian President Toomas Hendrik Ilves (left), with intelligence officer Eston Kohver in 2010. Kohver was arrested by Russian police on spying charges, but Estonian officials called it an illegal kidnapping. AP hide caption

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President Obama stands alongside British Prime Minister David Cameron, center left, NATO Secretary General Anders Fogh Rasmussen, second left, Croatian President Ivo Josipovic, left, Romanian President Traian Basescu, right, and RAF Group Captain David Bentley, second right, during a flypast at the NATO summit at the Celtic Manor Resort in Newport, Wales on Friday. Jon Super/AP hide caption

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President Obama sits with (from left) French President Francois Hollande; Ukrainian President Petro Poroshenko; British Prime Minister David Cameron; German Chancellor Angela Merkel; and Italian Prime Minister Matteo Renzi as they meet about Ukraine Thursday at the NATO summit at Celtic Manor in Newport, Wales. Charles Dharapak/AP hide caption

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Pro-Russian rebels prepare their weapons in the eastern city of Donetsk on Aug. 31. Russia's role in Ukraine has raised tensions between Russia and NATO to their highest level since the Soviet breakup more than two decades ago. Mstislav Chernov/AP hide caption

itoggle caption Mstislav Chernov/AP