Residents observe a police officer take up position during an operation at the Mare slums complex in Rio de Janeiro March 25, 2014. The federal troops have taken control of security operations in Mare slums complex to help quell a surge in violent crime following attacks by drug traffickers on police posts in three slums on the north side of the city, government officials said. (Photo by Ricardo Moraes/Reuters)

German field telegraph operators at work in Warsaw. (Photo by Hulton Archive/Getty Images). 1st October 1914

A man pushes a tub carrying children as he gets them back home after school at a flooded area in Duchang, Jiangxi Province, China, June 27, 2016. (Photo by Reuters/Stringer)

A period of relative stability has put the Iranian capital No1 on the list of world cities that have achieved biggest improvements in liveability over the past five years, as calculated by the Economist Intelligence Unit. The “most improved” top 10 contains some unexpected names. So, eg, the fourth place: Abidjan, Ivory Coast. Latest ranking: 128; Ranking five years ago: 131; Five-year index movement: 3.8%. Here: A supporter of Ivory Coast president Alassane Ouattara campaigns in a market in the city’s Koumassi quarter. (Photo by Sia Kambou/AFP Photo)

A dressed reveler looks up to the famous cathedral in Cologne, Germany, Wednesday, November 11, 2015. Thousands of carnival supporters celebrate the traditional beginning of the carnival season in Germany. (Photo by Martin Meissner/AP Photo)

In this Monday, October 5, 2015 photo, a thick blanket of early morning fog partially shrouds the skyscrapers of the Marina and Jumeirah Lake Towers districts of Dubai, United Arab Emirates. Dubai's rapid transformation from a desert outpost into one of the world's most architecturally stunning cities is mapped out in the Marina. Where just 15 years ago there was empty, flat land, today a bustling neighborhood thrives centered around a canal and an impressive skyline that pierces through the clouds. (Photo by Kamran Jebreili/AP Photo)

A six-year-old acrobat performs on the opening of a tourist festival in Bozhou, Anhui province, China May 3, 2015. (Photo by Reuters/China Daily)

In this photo taken Saturday, June 20, 2015, Mahmoud Abdulnabi tries out an oud, an Arabic instrument related to the lute, at his workshop in Baghdad, Iraq. “The oud is different than other musical instruments”, said Abdulnabi, who has crafted ouds played by some of Iraq's best known musicians, many of whom look down from headshots on the walls. “If you feel joyful, it can play your joy. If the circumstances are sad it can play your sorrow and... help to empty whatever is in your chest”. (Photo by Hadi Mizban/AP Photo)