Serbian police officers of the Special Anti-Terrorist Unit pose for a picture in their base outside Belgrade October 8, 2014. When the killing of an unarmed black teenager by a white policeman in Ferguson, Missouri, in August sparked sometimes violent protests, the response of police in camouflage gear and armoured vehicles wielding stun grenades and assault rifles seemed more like a combat operation than a public order measure. Some U.S. police departments have recently acquired U.S. military-surplus hardware from wars abroad, but there are many law enforcers around the world whose rules of engagement also allow the use of lethal force with relatively few restrictions. But for every regulation that gives police wide scope to use firearms, there is another code that sharply limits their use. In Serbia, police may use measures ranging from batons to special vehicles, water cannon and tear gas on groups of people who have gathered illegally and are behaving in a way that is violent or could cause violence, but they may use firearms only when life is endangered. (Photo by Marko Djurica/Reuters)

A Ukrainian woman begs Ukrainian President Petro Poroshenko to stop the bombing in Donetsk after shell hit the residential area where she lives, killing two civilians in Donetsk's Kyibishevsky district, on January 29, 2015. (Photo by Manu Brabo/AFP Photo)

Vladimir Putin looks at a model of the Tupolev TU-160 bomber, or Blackjack, that was presented to him on his arrival in Olenegorsk, Russia, August 16, 2005. Putin flew in the Tupolev TU-160 bomber and took part in the launch of cruise missiles in the Arctic north. (Photo by Reuters/ITAR-TASS/Presidential Press Service)

This photo provided by Joan Marcus on August 28, 2015 shows ballerina Misty Copeland in the musical “On the Town” at the Lyric Theatre in New York. Until September 6, 2015 Copeland will play Miss Turnstiles, a love interest for one of three sailors enjoying a few hours of shore leave in 1940s New York. (Photo by Joan Marcus via AP Photo)

The Soyuz TMA-18M spacecraft is transported from an assembling hangar to its launch pad at the Baikonur cosmodrome, Kazakhstan, August 31, 2015. The Soyuz is scheduled to blast off with Aidyn Aimbetov of Kazakhstan, Sergei Volkov of Russia and Andreas Mogensen of Denmark to the International Space Station (ISS) on September 2, 2015. (Photo by Shamil Zhumatov/Reuters)

Laura Paipo (L), first blind principal in Uruguay participates in a street race with her sighted guide and friend Amparo Bauter in Montevideo, October 4, 2015. Paipo, 55, who took office on August 10, 2015 became the first blind principal of a special school which specializes in the education of visually impaired students. (Photo by Andres Stapff/Reuters)

A supercell that spawned a tornado earlier makes it was east north of McLean, Texas on April 16, 2015 in Texas, United States. Huge hail, terrifying lighting and giant supercells the size of buildings – these incredible images depict a year in the life of a storm chaser. Shot in 2015 by photographer Mike Olbinski, 40, this series of pictures show both the beautiful and the extreme nature of weather in the United States. (Photo by Mike Olbinski/Barcroft Media)

A group of young men use a selfie stick to take a picture of themselves in shallow waters known as the first cataract of the River Nile outside Khartoum, Sudan, May 22, 2015. In Sudan, which faces insurgences in the western region of Darfur and along its border with breakaway South Sudan, as well as double-digit inflation and high unemployment, life goes on for young people in the capital Khartoum. (Photo by Mohamed Nureldin Abdallah/Reuters)