People in Mexico wave at U.S. Border Patrol agents on horseback patrolling the U.S.-Mexico border fence near San Diego, California, U.S., November 10, 2016. (Photo by Mike Blake/Reuters)

Ukrainian servicemen patrol near the humanitarian aid center in Avdiivka, Ukraine, Saturday, February 4, 2017. Fighting in eastern Ukraine sharply escalated this week. Ukraine's military said several soldiers were killed over the past day in shelling in eastern Ukraine, where fighting has escalated over the past week. (Photo by Evgeniy Maloletka/AP Photo)

A young migrant cries as she waits to cross the Greek-Macedonian border, near the village of Idomeni, Greece March 3, 2016. Thousands of refugees and migrants wait on the border between Greece and Macedonia and about 30,000 refugees and other migrants are stranded in Greece, with 10,000 at the Idomeni border crossing to Macedonia. (Photo by Marko Djurica/Reuters)

Girls cry as they get separated from their family at the border line dividing Macedonia and Greece August 21, 2015. At least 1,000 migrants and refugees pressed against Macedonian police lines on the Greek-Macedonian border on Friday and at least 10 people appeared to faint in the crush. People could be heard screaming and medical workers raced to treat those who passed out or were hurt. The crush ensued after police let several hundred through into Macedonia, having kept them out since Thursday under an emergency decree. (Photo by Ognen Teofilovski/Reuters)

Buildings in Nogales, Mexico (R) are separated by a border fence from Nogales, Arizona, United Sates, October 9, 2016. Building a wall along the U.S.- Mexico border has been a contentious subject in this year's U.S. presidential election. In parts of California and Arizona, a wall already exists. It runs across rocky deserts, flowing sand dunes and miles of agricultural land. (Photo by Mike Blake/Reuters)

A North Korean soldier throws a stone towards a photographer on the banks of the Yalu River in the North Korean town of Sinuiju, opposite the Chinese border city of Dandong October 21, 2006 in Sinuiju, Democratic People's Republic of Korea. (Photo by Cancan Chu/Getty Images)

A man walks next to a section of the wall separating Mexico and the United States, on the outskirts of Ciudad Juarez, Mexico, November 12, 2016. (Photo by Jose Luis Gonzalez/Reuters)

Cars pass a tank position close to the Russian border near the Ukrainian city of Kharkiv March 24, 2014. U.S. President Barack Obama sought support from European allies and China on Monday to isolate Russia over its seizure of Crimea, and Ukraine told its remaining troops to leave the region after Russian forces overran one of Kiev's last bases there. (Photo by Dmitry Neymyrok/Reuters)