A US Army M1A2 SEP tank participates in the Strong Europe Tank Challenge 2017 at the military training area in Grafenwoehr, Germany, 12 May 2017. Six North Atlantic Treaty Organization (NATO) nations and partner nations, Austria, France, Germany, Poland, Ukraine and the USA are taking part in this competing exercise. (Photo by Christian Bruna/EPA)

Syrian Internal Security Forces dance in celebration during their graduation ceremony, at Ain Issa desert base, in Raqqa province, northeast Syria, Thursday, July 20, 2017. Some 250 residents of Syria's Raqqa province are the latest batch to graduate from a brief U.S-training course that is preparing an internal security force to hold and secure areas as they are captured from Islamic State militants. (Photo by Hussein Malla/AP Photo)

Pfc. Remy Bouchard swaps a cigarette for an egg with a French Orphan near La Haye Du Puits, France on July 18, 1944, which was captured by American forces. The boy is only twelve years old. (Photo by Hugh Broderick/AP Photo)

Princess Diana relaxes on the sand during a visit to the beach on the Caribbean Island of Nevis January 4, 1993. (Photo by Mark Cardwell/Reuters)

Cao Ri and Xu Jiaqian have a wedding photograph taken in a pool at the Qianyishi Underwater Photography Studio in Beijing, China on September 7, 2017. (Photo by Thomas Peter/Reuters)

A participant in costume uses a mobile phone at a Halloween event in Kawasaki, south of Tokyo, Japan October 29, 2017. (Photo by Kim Kyung-Hoon/Reuters)

Spacewalker Mark Vande Hei took his own photograph during the first spacewalk of 2018, on January 23, 2018. These sky-high pictures are better known as “space-selfies”. (Photo by Mark Vande Hei/NASA)

Indonesian students go to school via a makeshift suspension bridge in Ngemplak, Boyolali regency, Indonesia on November 15, 2017. The lack of access to roads makes students every day to cross the dangerous waterways to go to school. (Photo by Arief Setiadi/Pacific Press/Barcroft Images)