_{Great Wall of China - By Jakub Hałun - Own work, CC BY-SA 3.0}

This is not the first time in history that a superpower notable has called for a long, protective wall to hold back unwanteds, although perhaps the first time the leader has been so demagogic as to assert that he will have the unwanteds pay for it.

In 117, Hadrian became the emperor of Rome as it was reaching its zenith. His 73-mile, east-west wall across Great Britain took six years to complete and demarcated the northern limit of the Roman Empire in its province of Britannia. Historians disagree about the reasons for the wall. When he acceded to the throne, there was much rebellion throughout the Empire, not only in Britannia, but also in Egypt, Judea, Libya, and Mauritania, and he wished to shore up defenses on the frontiers. Clearly one of the reasons was to keep out the barbarian Picts, Celts, and others. Secondly, it served as a visible and prominent symbol of Roman power—manned with Roman forces, turrets, watchtowers, white-washed and gleaming in the sun. Also in question was its effectiveness, although it appears there was no major invasion from the north while the wall was manned.

Like Hadrian’s Wall, the Great Wall of China was to serve as a barrier between the civilized and uncivilized world to its north and also as a symbol of the power of the Qin Dynasty, the first Chinese empire. The first part of the wall was built in the 3rd century BC in response to the threat posed by the nomadic Xiongu people on China’s northern frontier. Emperor Qin Shi Huangdi sought to strengthen fortifications to keep the nomads out. He sent Meng T’ien with 100,000 men to address the challenge. The Han Dynasty historian Sima Qin describes how Meng “seized control of all the lands south of the Yellow River and established border defenses along the river….” “The whole line of defenses stretched over ten thousand li [about 3300 miles]….” Over the centuries the wall was expanded so that it eventually stretched to over 13,000 miles. In addition to a defensive fortification it also served the purposes of border control and migration control. Effective at times as it might have been, it certainly did not prevent the conquering of China by the Mongols in the 13th century or the Manchus in the 17th.

The most heavily militarized border in the world today is the De-militarized Zone which divides North and South Korea, where I served as an Army officer 1981-82. It is 160 miles long and about 2.5 miles wide. I was stunned the first time I traveled to the DMZ and surveyed the extensive fortifications—fences, minefields, ramparts, watchtowers, and even hooks submerged in the Imjin River. At that time South Korean boats were patrolling the river because an infiltrator had been detected shortly before my arrival. Clearly a tremendous amount of resources has been expended by both sides on these border fortifications. At present, the United States maintains about 28,500 military personnel in support of South Korea. Despite all the effort, North Korean tunnels have been found under the DMZ in 1974, 1975, and 1979. At the time I served there, intelligence sources estimated that the North Koreans had dug probably over 20 such tunnels, still undiscovered.

These examples suggest that unless a country is prepared to expend great resources in building and policing a wall, human ingenuity finds ways to breach it in proportion to the strength of the incentives—empire, more grazing land, conquest, and now economic opportunity and wealth from drug money. And the more impermeability a country seeks with its wall, the greater the expenditure of resources.

Unlike the above examples, the 1950-mile, US-Mexican border does not present a military threat to the United States, although it does present a perceived security threat. Nonetheless, the examples instruct us to question a potential wall’s effectiveness and to ensure the benefits justify the resources expended.

Donald Trump has made a campaign pledge to build a wall, some 1,000 miles long, and to “find and dislocate tunnels and keep out criminal cartels.” However, this pledge is based on an apparently false premise—that technologies exist or are within reach to find the estimated hundreds of tunnels that traverse the border. Using current tunnel detection technology, David Shaw, a special agent in charge of Homeland Security Investigations in San Diego, stated, “We’ve never found a tunnel using them.”

Since the first tunnel was found in 1990 in Douglas, Arizona, border officials have found nearly 200. Allowing that many more may exist, R. Gil Kerlikowske, the US Customs and Border Protection commissioner, recently indicated, “But you don’t know what you don’t know.” Kevin Hecht, a Border Patrol tunneling expert in Nogales, Arizona, commented on the tunnels. “For every tunnel we find, we feel they’re building another one somewhere, and they get more creative in concealing it.” “Until there is some device on the market to help us accurately detect them, we just won’t know.”

Robert Frost wrote about walls between neighbors in his poem, “Mending Wall.” “Something there is that doesn’t love a wall, That wants it down,” Frost says to his neighbor. In his election-season pandering to conservative, dominant-culture Americans, Trump clearly embodies Frost’s neighbor who resists the needlessness of the wall on certain parts of their border—where Frost’s apple orchard borders his neighbor’s pines and where there are no roaming cows to keep in.  However his neighbor, holds fast and says again, “Good fences make good neighbors.”