HONOLULU – Call this the Year of Border Walls. In 2015, Estonia, Hungary, Kenya, Saudi Arabia, and Tunisia all announced or began the construction of barriers on their frontiers. We may live in an era of globalization, but much of the world is increasingly focused on limiting the free movement of people.

At the end of World War II, there were only five border walls around the world. Today, according to Elisabeth Vallet of the University of Quebec at Montreal, there are 65, three-quarters of them built in the past 20 years.

And in the United States, Republican presidential candidates are promising more. The Republican frontrunner, Donald Trump, has repeatedly proposed building a wall along the entire border with Mexico. And on a Sunday morning talk show, another Republican candidate, Wisconsin Governor Scott Walker, described building a wall on the US-Canada border as "a legitimate issue for us to look at.”

And yet existing border walls are neither cheap nor effective. Israel’s wall in the West Bank cost more than $1 million per mile to construct. According to US Customs and Border Protection, building and maintaining the existing 670 miles of border fencing on the US-Mexico border would cost $6.5 billion over the barrier’s expected 20-year life cycle. At this price, fortifying the remaining 1,300 miles of the Mexico border would cost more than $12.6 billion. Erecting a wall along the 5,525-mile border with Canada would cost almost $50 billion and would cut through an airport runway, an opera house, homes, and businesses that currently straddle the border.

Nor is there much evidence that border walls work as intended. To be sure, prisons demonstrate that short, well-guarded walls can be extremely effective at preventing movement. But even prison walls are only as effective as the guards who ensure that they are not breeched, and guards can be susceptible to bribes. The recent escape of the drug cartel leader Joaquin "El Chapo” Guzman from a Mexican prison highlights another vulnerability of border walls: tunnels. Since 1990, the US Border Patrol has found 150 tunnels beneath the US-Mexico border.

Those with money will always be able to cross borders using fake documents, bribes, or innovative infrastructure.

Indeed, fortified frontiers are most effective at stopping poor migrants and refugees. And even then, rather than preventing migrants from entering, fortifications all too often funnel them toward more dangerous crossing points. The result is a mounting toll of predictable deaths. The International Organization for Migration estimates that from 2005 to 2014, some 40,000 people died attempting to cross a border.

Unlike prison perimeters, borders can be thousands of miles long, which makes them difficult to monitor properly.

The US employs more than 20,000 Border Patrol agents; but even if they were all on duty at the same time, each would each need to guard a 1,700-foot section of the border.

Of course, equipment like cameras, motion sensors, drones, helicopters, and vehicles allow agents to watch long sections of the border. But the necessity of monitoring border walls points to one of the fundamental truths about them: historically, most have proved to be pretty useless. The most famous sections of the Great Wall of China were overrun within a few decades of their construction. When Germany invaded France in WWII, it simply went around the Maginot Line. The Berlin Wall fell within 30 years of its construction.

Indeed, border guards and their equipment can be equally effective without a physical barrier. At best, walls and fences only slow people down, making them a poor investment from a security standpoint. They are similarly ineffective from a military perspective. Missiles and airplanes can fly over them and tanks can smash through them.

Reece Jones, a professor of geography at the University of Hawaii 

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