Now that we seem intent on building a wall, it might behoove us to look at a few other wall builders and how their constructions worked out. Although I have never been one to believe history repeats itself, it certainly does follow patterns and processes.

We could start with the Roman Emperor Hadrian and the wall he constructed across Britain in the second century. It was designed to protect civilized Roman Britain from the Picts, who they saw as savages, in what is today Scotland. Although parts of the wall still exist today — it is a UNESCO World Heritage site — its usefulness was short lived. It appears to have kept the Picts out, but by the third century Rome had a lot more trouble on its hands than wilding Brits. Turmoil on the great Asian steppes had put vast numbers of people on the move, who ultimately would overrun the empire from all sides.

We could use the example of the Berlin Wall. Here was a wall made of cement and brick that completely encircled and cut off West Berlin from the rest of the world. This wall held up well for about 28 years, though it did require an entire division of troops along with machine-gun emplacements, minefields and a shoot-to-kill order. It was effective at keeping East German residents from emigrating to the West, but it could not keep out the real enemy — a free market economy.

The biggest of the wall builders, and President Donald Trump’s favorite example, would be the Great Wall of China, built during the Ming Dynasty of the 14th century. This 5,000-mile-long wall dwarfs the proposed wall along the U.S.-Mexico border, and it was built through terrain that is far more daunting. If the Chinese could build a wall of stone over mountains and through deserts large enough that two chariots could be driven side by side along its top, we should be able to build a 1,300-mile wall along the Mexican border. Of course, building the wall impoverished the dynastic treasury and cost an estimated 400,000 lives in its construction. It still might have been considered a success, except that it never kept the Manchu invaders out, and by the beginning of the 16th century they overran and conquered all of China.

Oddly enough, the defensive failure of the Great Wall was not the worst outcome. At the time of its construction, the Chinese had also begun a series of deep sea voyages across the Indian Ocean to the coast of Africa. On one voyage, they went as far as East Africa. They did this in fleets of hundreds of ships, in voyages that lasted for years with massive logistical support. The admiral, Zheng He, is a national figure in Chinese history. All of this was done 75 years before Christopher Columbus crossed the Atlantic. Unfortunately, building the wall cost so much that these voyages had to be discontinued.

Imagine what the world would have looked like if Ming emperors had decided to put their money in exploration instead of the wall and Zheng He continued around the Horn and up the west coast of Africa — or across the Atlantic.

Here lies the true cost of wall building. Wall builders act out of fear. They worry about their property and fear that the immigrants are going to take it away or their own citizens are going to emigrate with it. They spend their resources trying to lock down what they have, and in doing so, forgo their investment in the future. They put their faith in the wall instead of facing the world on its own terms. They never see the vandal tribes coming from the other direction until it is too late. They don’t experiment with new ideas, and so the free market overwhelms them. They spend their time and effort piling rocks on top of each other instead of opening up the world.

In economics there is a concept called “opportunity cost.” Opportunity cost simply means that the true value of anything is not what you paid for it, but rather, what you had to forgo to get it. We should all fear what will be the opportunity cost of building this wall.

Alan Haley teaches economics and writes about Maine life. He lives in Skowhegan, and he can be contacted at ahaleywshs@gmail.com.