A Saudi soldier fires a mortar towards Houthi movement position, at the Saudi border with Yemen April 21, 2015. (Photo by Reuters/Stringer)

Soldiers take part in a drill during a heavy snow in Heihe, Heilongjiang province, February 26, 2014. (Photo by Reuters/China Daily)

A line of soldiers firing handguns during a training session, circa 1920. (Photo by Keystone). P.S. All pictures are presented in high resolution.

A South Sudanese People Liberation Army (SPLA) soldier patrols the streets in Malakal on January 21, 2014. (Photo by Harrison Ngethi/AFP Photo)

Soldiers, including two recruits who have brought some chickens, at Victoria prepare to board the train for the battle front. (Photo by Hulton Archive/Getty Images)

Soldiers try to attach the balloon to their car, June 1944. (Photo by Jared Enos/Mediadrumworld.com)

A German rifleman beside the corpse of a French soldier in a trench at Fort Vaux, France, 1916. (Photo by Hulton Archive/Getty Images)

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