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

Israeli soldiers get ready to launch the Skylark drone during a drill on January 16, 2012 near Bat Shlomo, Israel. (Photo by Uriel Sinai/Getty Images)

Islamic female students practice the ancient Thai art of “Krabi Krabong” taught as a sport at the Darunsat Wittaya school on August 16, 2011 in Saraburi, Thailand. (Photo by Paula Bronstein/Getty Images)

Three female friends dancing in nightclub. (Photo by Cultura Creative (RF)/Alamy Stock Photo )

Sleeping Anthidium. Female Anthidium punctatum; Size: 7 mm. (Photo by John Hallmén)

A National Army of Colombia soldier, who was wounded by a mine, swims at a pool in Bello, municipality of Antioquia, March 25, 2015. (Photo by Fredy Builes/Reuters)

American soldiers leaping from an armoured personnel carrier during exercises in Korea, 17th March 1954. (Photo by Three Lions)

A soldier walks on the bank of the river in central Pyongyang, North Korea on April 16, 2017. (Photo by Damir Sagolj/Reuters)