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)

Fighters from the Democratic Forces of Syria prepare their weapons in the southwestern countryside of Hasaka, Syria February 17, 2016. Picture taken February 17, 2016. (Photo by Rodi Said/Reuters)

Cha Huilan, a 40-year old Lisu woman, and her daughter leave Lazimi village with a zipline across the Nu River in Nujiang Lisu Autonomous Prefecture in Yunnan province, China, March 24, 2018. Chinese mountain villagers, cut off from shops and churches by a raging river, use a zipline to cross its violent rapids and jagged rocks. (Photo by Aly Song/Reuters)

Stuart Haygarth hangs his spectacle chandelier at The Lighthouse, Scotland's national Architecture Design Centre on November 29, 2007 in Glasgow, Scotland. The 7ft chandelier is made of over 1000 pairs of glasses, and is one of a range of products the designer has made out of recycled products. (Photo by Jeff J Mitchell/Getty Images)

India's Central Reserve Police Force personnel take part in a rehearsal for the Republic Day parade on a cold winter morning in New Delhi January 8, 2014. (Photo by Ahmad Masood/Reuters)

A man rides his bike along a controversial bike lane on Prospect Park West on August 17, 2011 in the Brooklyn borough of New York City. In what is being viewed as a victory for the Bloomberg administration, a judge on Tuesday dismissed an effort by Brooklyn residents to remove a bicycle lane installed by the city on Prospect Park West. Mayor Michael Bloomberg, along with his transportation commissioner, Janette Sadik-Khan, has attempted to make New York more bicycle and pedestrian friendly with numerous bike and pedestrian lanes around New York City. The effort has angered some in the city who see the lanes as adding to traffic, taking up parking spaces and a danger to pedestrians. (Photo by Spencer Platt/Getty Images)

A soldier helps another with her ear protection at the Terningmoen Camp in Elverum, Norway on March 23, 2017. Soldiers demonstrate their skills and tactics during a contract drill as they train to become part of the world's first all-female special forces unit, the Jegertroppen or “hunter troops”. (Photo by Carolina Reid/NBC News)