Russian soldiers write home while serving on the Eastern Front during World War I, circa 1915. (Photo by Hulton Archive/Getty Images)

In this November 2, 2017 photo, Mathew Fulkerson and his wife Leigh Ann pose at their Subterra Airbnb located in a former underground missile silo base near Eskridge, Kan. It was designed to house a nuclear warhead six decades ago – but now, this Cold War silo is the perfect spot for a mini break at just $133 a night. (Photo by Thad Allton/The Topeka Capital-Journal via AP Photo)

An inmate covers his head as he passes the time inside Quezon City Jail in Manila, Philippines October 19, 2016. Nearly 2,300 drug users and dealers have been killed in police operations or by suspected vigilantes since Duterte took office on June 30, according to the Philippines police. (Photo by Damir Sagolj/Reuters)

“Shortly after he arrived, Van Agtmael witnessed the aftermath of this suicide bombing at a cafe that soldiers frequented in Mosul. Nine people died and 23 were wounded”. (Photo and caption by Van Agtmael/Harrison Jacobs/Magnum Photos)

These Arab recruits line up in a barracks square in the British Mandate of Palestine, on December 28, 1940, for their first drill under a British solider. Some 6,000 Palestinian Arabs signed up with the British Army during the course of World War II. (AP Photo)

Tajammu Al-Ezza brigade fighters react while firing a Grad shell towards forces of Syria's President Bashar Al-Assad stationed in Salhab village, Hama countryside, May 7, 2015. (Photo by Mohamad Bayoush/Reuters)

A man holds a cow at the cattle market in Maiduguri, Nigeria, March 9, 2016. A Nigerian government push to strangle the Boko Haram insurgency has shut down the cattle trade that sustained the city of Maiduguri, leaving many residents with no livelihood, including many of the two million people displaced by the war. In recent months the army has taken back much of the territory lost to the jihadists during the five-year insurgency. (Photo by Afolabi Sotunde/Reuters)

Canadian soldiers from the Royal 22nd Regiment take up a position during a non-combative extraction operation in a simulated village as part of Rim of the Pacific (RIMPAC) 2016 exercise held at Camp Pendleton, California United States, July 11, 2016. (Photo by Mike Blake/Reuters)