Donald Trump’s 2016 presidential campaign slogan, “Build That Wall,” has achieved iconic status as a shorthand for the cultural divide between those Americans who see immigration as integral to our national character and a southern boundary wall as immoral and those who view immigration as a threat to our way of life and an open border with Mexico as a dagger aimed at the nation’s heart.

Whichever side of the debate you’re on, however, it’s important to understand that protective boundary walls are neither unique to the United States nor to our era. They have been built on many occasions and in many places with varying degrees of success and failure. The walls of the past contain lessons for the would-be wall builders of today, including President Trump.

Thousands of miles of walls have been erected over four millennia of human history. Most have been abandoned and, to the extent they’ve survived, appear as crumbled, half-buried remnants. They’ve been fashioned from materials ranging from primitive sun-baked bricks, tamped-down earth, and wooden palisades to more advanced masonry blocks, concrete slabs, and razor-wire topped metal fencing. Most importantly, they’ve served the legitimate purpose of enhancing security but have also inevitably contained fatal laws.

In his recent book. “Walls: A History of Civilization in Blood and Brick,” David Frye, an archaeologist and professor of ancient and medieval history, tells the compelling story of how walls have cocooned and undermined civilizations.

Through most of human history, walls were used to protect settled farming communities and cities (“civilization”) from the ravages of war-like nomadic peoples (“barbarians”) sweeping in from semi-arid grasslands (steppes). The greatest of the steppes, the Eurasian, runs like a long cable some 5,000 miles from the Carpathian Mountains in the Balkans to Manchuria in the Far East. Nomads roaming these vast regions lived off the land, scavenging wild grains, fruits and roots, herding and hunting animals for milk, meat, fur, and bone implements, and raiding settled communities in search of loot and slaves.

Settled communities made long-term investments in elaborate irrigation systems, large urban buildings, and cultivated fields, which required protection from the dangers of invasion. As they became bigger, settled communities turned into kingdoms and empires, whose leaders employed two methods for their defense.

The first was to maintain a professional army, consisting of a small segment of the population who were trained, equipped and maintained at state expense. The second was to build walls to shield settled areas against invasion. Behind these protective walls, a growing and increasingly affluent civilian population could specialize and branch out into all the manifestations of civilization, including commerce, architecture, astronomy, medicine, art, theater, philosophy and temple-based religion.

The more “civilized” a society, however, the more detached its inhabitants became from the practice of war and the need for constant vigilance against ever-present threats lurking outside its walls.

Periodically, smaller nomadic groups coalesced into larger hordes around a forceful, charismatic leader and swept in waves over or around the walls that were meant to keep them out. The names of these invaders, such as the Huns, the Mongols and the Ottoman Turks, are seared into civilized memory.

What lessons does history offer as to whether walls provide an effective defense against external threat? The only answer I can glean from Frye’s book is: It depends!

Here are some of the factors that seem to me to have made a difference in the usefulness of walls throughout history and their implications for the U.S. today:

Walls have to protect against the right threat.

Elaborate walls built in Central Asia against nomadic marauders from the Eurasian Steppe were north facing, the wrong direction to fend off the onslaught of Islamic armies attacking out of the south from the Arabian peninsula beginning in the 7^th century A.D. Consider that the U.S. intelligence community today regards China, Russia and North Korea as our major threats, not Mexico. The vast majority of migrants crossing the Mexico-U.S. border are refugees from crime and poverty, not criminals, much less armed invaders.

Walls need an economically vigorous society to maintain them.

The ancient Romans were great wall builders. However, as the empire began to decay from within and pressure from barbarian invasions ramped up, Rome was unable to recruit enough legionaries to defend the walls and increasingly turned to auxiliary soldiers, often drawn from the same barbarian population which threatened those walls, to man them. By the 4^th century AD, these defenders proved unreliable at keeping the invaders at bay. Consider that today, due to federal budget deficits, the U.S. Border Patrol is understaffed and has difficulty just policing the country’s designated ports of entry which are already protected by barriers.

Walls need to be designed to be technologically effective. In 1453, the Ottoman Turkish leader Mehmed II used gunpowder cannons, a new technology, to breach the seemingly impregnable 1,000 year-old walls surrounding Constantinople, the last remaining stronghold of the Eastern Roman Empire. The Democrats’ current characterization of Trump’s proposed “big beautiful” solid wall as “medieval” is essentially correct. A solid wall can be tunneled under, flown over, breached with explosives, or simply circumvented.

Walls should never lead to complacency about the security of the society they are built to protect.

Before World War II, the French sat behind the supposedly impregnable Maginot line, a string of reinforced concrete bunkers armed with artillery, which were expected to protect their eastern border against any German invasion. The Germans invaded anyway in May 1940, end-running the Maginot Line by attacking through Holland, Belgium and the Ardennes Forest, and crushed French military resistance in just six weeks. Relying on the Maginot Line, the French had neglected their mobile force capability, particularly tanks and planes, which would have helped them repel the Germans. Currently the U.S. faces a far greater threat from aggressive cyberwarfare being waged by China, Russia, North Korea and Iran, than from any physical invasion from our southern border, yet the cyberwarfare threat is given short shrift by Trump.

In my view, history suggests that building a wall across our entire border with Mexico is unlikely to enhance our security, because it would be the wrong wall, in the wrong place, at the wrong time.