Donald Trump isn't the first leader to want to put up a border wall.
Defensive barriers have been around since the beginning of time, and the first walls sprung up 11,500 years ago in Turkey to wall off cities from attack.
"Throughout history, walls have had both a symbolic function as much as a mechanical barrier," said John Linnell, senior researcher from the Norwegian Institute of Nature Research. "This goes all the way from your garden fence to the Great Wall of China."
Not all walls have done what they were designed to do, and, most like the Great Wall of China and Berlin Wall, have ended up as tourist attractions. Historians and researchers advise there are lessons to be learned from every major wall including their effects on migration, the environment and history.
The Great Wall of China
Arguably one of the three most famous walls in the world, and one of the oldest, the Great Wall of China was built as early as the third century BC to defend against invaders. The wall is 5,500 miles long, almost three times that of the US-Mexico border.
"The Great Wall of China was built just as much to keep the Chinese people in as to keep the Mongols out," Linnell said.
The wall rose 50 feet in some areas, which is more than double Trump's proposed border wall. The Ming Dynasty was the last to build parts of the Great Wall for defense in 1474. In the 1950s, parts of the wall were rebuilt but only for aesthetic reasons. Today, it is a major tourist attraction and UNESCO World Heritage site.
The Roman Wall
The Roman Wall, also known as Hadrian's Wall, was built for the Roman emperor Publius Aelius Hadrianus in 122 AD. It was built by the Romans in Britain to split the north from the south -- which was a territory of the Roman Empire. It stood 15 feet tall (just three feet shorter than Trump's proposed wall), 73 miles long, and took six years to build. Historians say the impetus for building the wall were threats from other cultures in the north, but it's unclear how much of a threat there really was. Other historians suggest the wall may have simply been used as a statement to exercise the power of the Roman Empire -- a.k.a. a power play or ego boost.
The wall was finished in 128 AD, and 10 years later, Hadrian died. A new emperor took his place and didn't think much of the wall. Soon after the Roman's power in the region started to decline and in 410 AD, the Roman rule ended.
"The wall didn't prevent the loss of Britain," said Daniel Jütte, Associate Professor of History from NYU. "And it didn't prevent the collapse of the Roman Empire."
Today, the Roman Wall is a World Heritage site.
The Berlin Wall
The Berlin Wall was built in 1961 for one specific purpose: to encircle West Berlin and keep anyone from going in, or out.
Historians point to the wall as the literal "iron curtain" during the Cold War era. The 15-foot-wall was heavily guarded and the only way to legally pass through were through checkpoints. In the 28 years of the wall's existence, there were about 5,000 successful escapes into West Germany but a couple hundred people also lost their lives trying to cross.
Some people resorted to tunnel digging to get under the wall, a technique also used with the wall on the US-Mexico Border.
"It's now a tourist attraction -- whatever is left of it," Jütte said. "Like every other wall in history, it hasn't outlasted the course of time."
The fall of the Berlin Wall in 1989 was symbolic of a new border-less world which encouraged the flow of people and goods.
Today, the Berlin Wall has been gone longer than it stood.
US-Mexico Border Wall
That borderless new world changed with the September 11, 2001 terror attacks, and walls started to go up again. In 2006, the Secure Fence Act was passed and the first fences went up on the US-Mexico border. According to US Customs and Border Protection, 1,071,972 people were apprehended in 2006. The following year, construction of the fence began and apprehensions dropped by over 200,000. The wall has also reduced animal migration. Endangered species like ocelots in the Rio Grande Valley have lived along the border for nearly a century. There are only 52 documented Ocelots in the US and they are all in southeast Texas.
"The Rio Grande delta is one of the fastest developing tracks of property in the United States," said Dr. Michael Tewes from Texas A&M University, who has been studying ocelots for 35 years. "So that's going to be a threatening aspect for the cat population there, and their ability to disperse and occupy other areas."
The Trump administration has proposed a border wall that would run right through the Santa Ana Wildlife Refuge in Texas -- prime habitat for ocelots.
The next great migration was the refugee crisis in Europe. In 2015, migrants started fleeing Africa and the Middle East in record numbers. Hungary closed its borders which left the border between Slovenia and Croatia wide open. In response, Slovenia built a 416-mile razor wire fence. Researchers from the Norwegian Institute of Nature Research say the transnational fence could wreak havoc on the wildlife known to stretch both borders.
"Wildlife is always going to be an innocent victim of a border fence," Linnell noted.
Three endangered species that use the border as a migratory path: the brown bear, gray wolf and the lynx could suffer population loss because of their inability to breed. The lynx -- like the ocelot -- is threatened by its small population and degree of inbreeding which becomes exacerbated by a physical barrier.
The fence "was an immediate reaction to an unprecedented movement of people," Linnell said. "People are probably still leaving Syria but there are simply not making it as far as Europe."
Nearly all of the pre-9/11 walls have something in common -- they either no longer exist or are no longer used as a defensive border.
"We really didn't see this whole issue coming," Linnell said. "I don't think any of us anticipated the extent of the problems that were going to come in the 21st century. We thought we has left all this behind at the Cold War, but in reality, the total opposite was happening."
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