The views presented herein represent those of the author, and do not necessarily reflect those of the U.S. government, Department of Defense, or the U.S. Air Force.

In 2016, Donald Trump campaigned for the presidency of the United States, largely on a platform of increasing the nation’s security, particularly along the southern border. One of his signature campaign promises was the construction of a massive wall along the entire U.S.-Mexico border, ostensibly to be paid for by Mexico, that would effectively halt illegal immigration across the largely undefended border. Trump’s call for a wall reflects his belief that such a construction would effectively halt the movement of undocumented immigrants from Latin America into the United States. He claimed that it would lead to a major curtailment of drug smuggling, human trafficking, and the movement of criminals into U.S. territory. In 2018, when Congress proved unwilling to authorize the funds necessary to commence large-scale construction of the wall, Trump threatened to veto any spending measure he received until the wall funds were added to the budget. As a result, the federal government endured the longest partial shutdown in the nation’s history, with most government agencies effectively furloughed for five weeks. One of the fundamental questions that confronts both supporters and detractors of the wall plan is the simple question: do walls work to stop large-scale migration?

Historically speaking, walls can have a significant effect upon the movement of large populations, whether attacking military forces or nomadic migrant populations, but only if they are backed by a large enforcement capability and a willingness to use force in defense of territory. By definition, walls offer a passive defense, although they may enable the activities of defenders stationed along and behind them. Their presence might slow down an aggressor enough to allow an active response, particularly if the wall’s construction includes elevated observation points. Most walls also act as a screen, making anyone’s attempt to cross or breach inherently uncertain of what to expect on the other side, and giving an inherent advantage to the defender. In general, any wall can be defeated, by going over, under, or through, although such activities might require specialized equipment and skills, particularly for the first individuals to cross. In principle, walls serve to keep others out, or to hold a specific group in place, although they have been utilized for point defense for as long as humans have been inhabiting specific locations. Often, the level of investment required to defeat a wall is more than the crossing is worth, making defensive walls an effective deterrent. Modern commentators tend to compare the planned Trump wall to existing structures in Israel—but a better comparison might be drawn to much older (and longer) walls.

Ancient Walls and Frontier Defense

There have been two truly great wall-building civilizations in human history, and a host of nations that have attempted to use the large-scale construction of walls to control the movements of populations. In many ways, the history of walls reflects the history of human civilization, as their construction both consumes the resources of a state and serves to protect the state that funds them. Walls can be an important element in national defense—the resources invested in their construction represent start-up costs, which are then returned to the state through the prevention of raids, major attacks, and the reduced cost to defend frontiers. However, in order to be effective in more than a simple deterrence fashion, walls need to be backed up by mobile guardians who possess the will to use deadly force in their defense. Otherwise, they are expensive to construct, difficult to maintain, and readily breached or bypassed. They might somewhat channel the movement of large populations of unmotivated migrants, but they will do little to obstruct determined individuals and groups from crossing. At best, when approached by a highly motivated group that wishes to cross a frontier, a wall might serve to channel or divert their movement to the easiest crossing points under the right circumstances.

The first great wall-building society in human history was the Roman Empire. The Romans developed extremely sophisticated engineering techniques, allowing many of their edifices to remain largely intact until the twenty-first century, and possibly beyond. At first, Roman wall-building was largely confined to point defense, including the construction and continual improvement of the walls surrounding the city of Rome. Those walls protected the city for centuries, allowing its inhabitants to huddle behind them in periods of invasion and hostile occupation. In 216 BCE, Hannibal turned away from an attack on the city after destroying the Roman army at Cannae, largely due to the imposing defensive walls he would have to overcome.

When the Romans moved into the British Isles, they managed to conquer the southern region, but found it extremely difficult to move into the northern highlands. Rather than continue their fruitless attacks for an area that yielded little bounty, the Romans chose to construct a wall across the entire width of Britain, completed in 128 CE and eventually known as Hadrian’s Wall. This fortification, portions of which still exist, stretched 73 miles and was protected by a series of fortified points.  While it was not tall enough to prevent individuals from scaling it to cross almost at will, it made any effort to supply a large force that crossed it almost impossible. In order to invade Roman territory south of the wall, the northern tribes would need to first destroy a significant portion of the wall, and undertaking that would allow the Romans to rally their local forces for defense. In 154 CE, the Romans completed the 39-mile Antonine Wall, which served to push their frontier further north—but it proved too costly to defend against continual attacks, and was abandoned just eight years after its completion.

Although Hadrian’s Wall is the most famous Roman frontier fortification, it is not the only such construction. The Romans tended to build walls at the limits of their imperial expansion, both as a symbol of Roman power and as a very real defensive enhancement. Roman walls of substantial length were completed in North Africa, the Balkans, and the Middle East. The Fossatum Africae, which was probably also ordered by Emperor Hadrian, stretched more than 750 kilometers in an arc southwest of Carthage, clearly delineating Roman territory in the region. Although there were discussions of building similar walls as a means to confine the Germanic tribes, no significant attempt to wall off Roman possessions in central Europe ever came to fruition. After the Roman Empire split into two portions, the wall-building legacy largely transferred to Constantinople. The capital of the Eastern Empire was protected by enormous walls that served to dissuade any serious threat for more than ten centuries. The Anastasian Wall, built in the late fifth century, stretched 40 miles in an attempt to protect the western approaches to Constantinople.

The Great Wall of China stands today as one of the largest human undertakings in history, although contrary to popular opinion, it cannot be seen from space. It represents the culmination of centuries of effort, beginning with the first wall built by the state of Qi during the Warring States period, in the seventh century BCE. Portions of that wall are still visible today, more than 2,500 years later. The first emperor of a unified China, Qin Shi Huang, ordered a significant expansion of the walls in the third century BCE, and his successors continued to improve and expand the defensive fortifications for centuries. In 1368, Zhu Yuanzhang founded the Ming Dynasty, and commenced construction of the Great Wall. Over the succeeding three centuries, the wall was expanded and fortified, until it stretched over 13,000 miles and boasted more than 25,000 towers, each protected by a permanent garrison. Although the wall was not impenetrable, it eliminated the threat of a large-scale Mongolian invasion, allowing the Ming Dynasty to focus upon consolidating its power behind the wall.

Archaeologists have discovered a significant number of long border walls, most essentially forgotten by history. For example, the Great Wall of Gorgan runs 120 miles from the shores of the Caspian Sea to the mountains in northeastern Iran. It might be as old as the fifth century CE, and appears to be designed to protect the Sassanians from the White Huns. Thirty major forts protected the wall, each large enough to hold a significant garrison, and it is likely that dozens of smaller outposts were included in the original construction. At approximately the same period, an unknown population constructed the Danevirke, a 28-mile wall that separates Jutland from the remainder of Denmark. There are hundreds of miles of walls stretching across Moldova, Romania, and Ukraine that are collectively called Trajan’s Wall, even though it is uncertain who was responsible for their construction. Unlike other Roman efforts, Trajan’s Wall is primarily an earthwork, with palisades still in evidence at some points along its length. It would have been much cheaper to construct and maintain than the masonry walls common to major empires, but also would have had less deterrent value.

Modern Walls for Ancient Problems   

Modern societies have also attempted to utilize walls as a means of curtailing the movement of large populations. A major modern walling effort was the Inland Customs Line, first established by the East India Company and then expanded by the British Empire to collect the salt tax in India. Unlike other walls made of artificial materials, the Inland Customs Line (also called the Great Hedge Wall) was composed primarily of extremely dense and thorny hedges, deliberately grown to over 12 feet in height. Like other walls, it served to slow down any attempted crossings and screen the movements of customs collectors and their escorts. Because its primary purpose was to prevent smuggling, the Inland Customs Line did not need to be defended in the same fashion as other frontier barriers, although violence between smugglers and customs enforcement patrols was extremely common. It proved to be an effective defense against salt smuggling, with thousands of smugglers apprehended and convicted each year by the 1860s.

One of the most famous symbols of the Cold War, the Berlin Wall stretched 96 miles and encircled West Berlin. Commenced in 1961, the wall served to prevent East Berlin citizens (and those living in the German Democratic Republic) from defecting to the West by slipping into the NATO-occupied city. Its graffiti-covered length soon came to represent the Iron Curtain, a visible artifact of the Cold War between East and West. Yet, most citizens in the West did not realize that the government of East Germany had commenced a much more ambitious project in 1952. The Inner German Border was designed to divide the two Germanys in a visible fashion, and remained an active project for more than three decades. By the 1980s, it ran more than 800 miles and was garrisoned by more than 50,000 border security troops. Approximately 1,000 citizens were killed attempting to cross the wall, which was backed by a fence line, concertina wire, a cleared free-fire zone, and hundreds of guard towers.

The state of Israel has also turned to physical barriers as a key part of its national defense strategy. The hostile border with the Gaza Strip has a 37-mile wall, coupled with a deep moat, designed to halt the infiltration of suicide bombers into Israel. Constructed from 1994-1996, the Israel-Gaza Security Barrier is credited with essentially ending the Hamas-directed suicide attacks campaign, forcing the Gaza-based terror group to focus instead upon rocket and mortar attacks. In 2000, participants in the Al-Aqsa Intifada tore down the barrier, leading to a strengthened barrier and a new rule of engagement for Israeli security forces: they are authorized to kill infiltrators on sight, rather than attempting to apprehend them. The Israeli West Bank Barrier has nearly 300 miles of completed wall, with plans for another 130. Like the Gaza Barrier, the West Bank Barrier has essentially reduced terror attacks originating in the West Bank to nearly zero. Portions of the wall are several meters high, creating a physical screen blocking sniper attacks upon Israeli motorists traveling the major highways in the region. Palestinian activists have accused the Israelis of using the physical barrier as a means to claim disputed territory in the region, while the Israelis claim that the boundary follows the topographical requirements of the area. These walls have resulted in significant international outcry, but have proven extremely popular with the domestic Israeli population, making it unlikely that they will be demolished in the foreseeable future.

The U.S.-Mexico Border

The Trump administration has vacillated somewhat on the exact composition, character, and funding source for its proposed wall. Although the government reopened in 2019 under a budgetary compromise, little money was set aside for border security. On February 15, President Trump declared a national emergency, which he saw as the necessary legal mechanism to allow him to shift funds allocated to executive agencies toward construction of the wall on the Mexican border. In making his announcement, he enumerated approximately $8 billion that would be allocated toward wall construction. When both houses of Congress voted on a bill designed to nullify the emergency declaration, Trump performed the first veto of his presidency, effectively leaving the national emergency in place. While it is too soon to judge whether his declaration will survive legal challenges from a number of directions, it is absolutely certain that the wall, by itself, will do little to deter smuggling and immigration. Thus, if Trump’s vision of border security is to become a reality, the construction of the wall is merely a first step in a much larger and more expensive process—previous attempts at walling off frontiers definitively show that it will require a major human presence and a willingness to use force, possibly lethal, before there will be a significant effect upon the flow of people and products across the border. Otherwise, any passive defensive fortification is likely to stand as an expensive monument to construction techniques that will do little to slow the movement of migrants across the border.