Romanian soldiers take part in a joint military exercise with NATO members, called “Agile Spirit 2015” at the Vaziani military base outside Tbilisi, Georgia, July 21, 2015. Georgian soldiers along with U.S. marines and platoon-size units from Bulgaria, Latvia, Lithuania and Romania are taking part in the exercises at the Vaziani military base outside Tbilisi. (Photo by David Mdzinarishvili/Reuters)

Ukrainian servicemen sit atop an armored personnel carrier (APC) at a military base in the town of Kramatorsk, eastern Ukraine, December 24, 2014. (Photo by Valentyn Ogirenko/Reuters)

NATO and U.S. flags flutter as U.S. Air Force F-22 Raptor fighter flies over the military air base in Siauliai, Lithuania, April 27, 2016. (Photo by Ints Kalnins/Reuters)

A man dressed as Santa Claus poses during a base flying event in downtown in Berlin December 6, 2014. Participants of the event hosted by entertainment agency Jochen Schweizer flew down 98 metres (322 ft.) from atop the hotel in the German capital. (Photo by Hannibal Hanschke/Reuters)

Katarina Stefanović is a passionate landscape photographer based in Belgrade, Serbia.

Ecuadorean spontaneous bullfighters participate in a popular bull festival called Las Canteras del Antisana at Pinantura village on the base of the Antisana volcano in Quito, November 28, 2015. (Photo by Guillermo Granja/Reuters)

A C-17 Globemaster III crew prepares an air refueling mission September 27, 2012, at Joint Base Charleston, S.C. (Photo by Airman 1st Class Matthew J. Bruch/U.S. Air Force photo)