Air Force One casts its shadow over homes as it comes in to land in Allentown, Pennsylania with U.S. President George W. Bush aboard in this October 1, 2004 file photo. (Photo by Jason Reed/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)

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 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)

Seals dressed in military uniforms swim during a show marking the 70th anniversary of the end of World War Two, at an aquatic park in the Siberian city of Irkutsk, Russia, May 9, 2015. (Photo by Evgeny Kozyrev/Reuters)

A National Transitional Council (NTC) fighter fires an RPG during a street battle in the 700 complex area of “Emarat” against Muammar Gaddafi's troops on October 8, 2011 in Sirte, Libya. NTC fighters say this is the final assault on Muammar Gaddafi's home town as they capture the main hospital, University and the Ouagadougou conference centre in the city. (Photo by Majid Saeedi/Getty Images)

Afghanistan's interior ministry special forces demonstrate their skills during a military exercise in Kabul, Afghanistan, Thursday, April 2, 2015. Afghanistan's security forces took over full responsibility for the country's security on January 1, 2015. (Photo by Massoud Hossaini/AP Photo)

Newly commissioned 2nd lieutenants throw their caps in the air as a team of F-16 Air Force Thunderbirds fly over the 2011 graduating class of the U.S. Air Force Academy on May 25, 2011 in Colorado Springs, Colorado. A total of 1,021 graduates received their diplomas in front of their families and dignitaries at the Academy's Falcon Stadium. (Photo by John Moore/Getty Images)