If the plan is to bet the United States’ national security on the siege-warfare technology of the ancient and medieval worlds, which is what a wall does, then our strategy has to be much more Byzantine.

I therefore reached out to various medievalists around the world to get their recommendations on how the United States can use technology that became obsolete in the 16th century to deter the murderous hordes of Trump’s fantasy amassing on the Mexican border. Just as the Pentagon undertakes a Nuclear Posture Review every few years, I did a Medieval Posture Review — and we’re slouches.

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To turn the 2,000-mile border into the walled fortress Trump desires, my experts suggest a medieval arms race as terrifying as the plague. Not only will we need a 30-foot “glorious wall” (Trump will like that term) with towers rising to 50 feet, but we’ll also need two more “curtain” walls, a moat and an earthen berm to keep away the invading migrants’ siege towers, ladders, battering rams and pole axes.

Atop the 10-foot-thick walls, crenelated parapets, screened by animal skins, will protect our archers from arrows and stones. The towers, rounded to deflect incoming boulders, will project outward — the better to hit illegal immigrants with enfilading fire from crossbows.

We’ll also need a full arsenal of ballistae to fire spears at the invaders and mangonels to launch pots of burning pitch at their siege weapons. Above all, we will need people — lots of them.

Leif Inge Ree Petersen, a siege-warfare expert at the Norwegian University of Science and Technology, explained to me that to do this right — and Trump wants only the best — we should match the “gold standard” in defensive walls: the Theodosian walls that protected Constantinople from George Soros-funded migrant caravans for 1,000 years, until 1453.

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Problem is, this wall had towers every 50 to 80 meters and required at least 20,000 people to defend its six-kilometer perimeter. To scale that up, Trump's border wall will need 51,200 towers and 10.7 million people to perform its various chores: pouring hot oil and dropping rocks on invaders, pushing away their ladders, firing flaming arrows, digging counter-tunnels to intercept invaders' tunnels and pulling ropes to operate the torsion catapults.

Another member of my war cabinet, historian Craig Nakashian of Texas A&M University at Texarkana, proposed a skeleton crew of the type the Romans used defending Hadrian’s Wall in the second century. But even that required some 15,000 people over 73 miles — so Trump’s border wall, by extrapolation, would still require more than 400,000 defenders. Siege warfare is labor-intensive. “You do kind of need people,” Nakashian explained.

Whether 400,000 or 10 million, the wall’s defenders are all going to need chain armor. And many must be trained in the most fearsome weapon of medieval times: the trebuchet. This monster, 100 feet tall and thousands of pounds, can hurl huge projectiles 1,000 feet.

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Cancel the F-35 contract. Lockheed Martin is going to have to build these suckers by the thousands. On the positive side, my experts said it will not be necessary to stock the moats with alligators (this was a Hollywood invention) nor to hurl diseased animal carcasses at the migrants (ineffective).

This is all going to cost well more than the $5.7 billion Trump has requested for the wall and requires mass conscription of civilians. But if we don’t win the medieval arms race, we risk a bloody repeat of the Sassanian Siege of Amida in 359, when Romans holding the city were overrun and killed (much as Trump claims illegal immigrants are doing to Americans). Without matching the enemy trebuchet-for-trebuchet, Petersen told me, “you have to sally out and burn the enemy’s trebuchets, and that can get nasty really fast.”

Of course, there is an easier way to protect our wall. We could use drones, ground-penetrating radar and that newfangled invention called “gunpowder.” But this would defeat the very purpose of building a wall in the first place: the frivolous novelty of using a fifth-century solution to a 21st-century problem.

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How else can we hope to recreate the esprit-de-corps that warriors of yore felt launching barrels of burning oil from their fortresses with a mighty battle cry?

“God help the Romans,” cried the ancients.

“In the name of God and St. Demetrius,” cried the Byzantines.

“Lord have mercy,” cried the medievals.

And from the parapets of Trump’s wall will come the sacred cry: Make America Great Again.

Twitter: @Milbank