The cause for all this is clear: President Trump refuses to see a funding bill passed without more than $5 billion allocated to the construction of the southern border wall that has become the white whale of his presidency — no matter the rounds of misinformation deployed to gin up support for it or the growing political cost the shutdown may exact on him and his party. According to new polling, while support for the wall has increased, a majority of Americans still oppose its construction and blame Trump for the shutdown.

“With Trump determined to deliver on his signature campaign promise of building a border wall and Democrats standing firm against what they view as an immoral and ineffective solution to illegal immigration, there is no end in sight to the dysfunction,” my colleagues reported.

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As the shutdown drags on, Trump and some of his allies may resolve to declare a national emergency to circumvent Congress and fund the barrier. It’s a move that even some Republicans believe would be “legally perilous” for Trump, The Post reports. But for the president’s narrow nationalist base — and the right-wing media talking heads who keep it in a state of semi-permanent agitation — the wall is a political battle Trump can’t afford to lose.

Trump’s insistence on the “wall” reportedly began as a kind of mnemonic, a tactic encouraged by advisers as a means to focus right-wing voters (as well as an easily distracted Trump) on his anti-immigration platform.

Wendy Brown, a political theorist at the University of California at Berkeley, argued that it was an ultranationalist siren song at a time when a sizable chunk of the American electorate faced rising housing and health-care costs, widening social inequity and a political system creaking under the weight of the country’s mounting divisions.

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“A ‘big, beautiful wall’ projects a different and simpler story,” Brown, the author of “Walled States, Waning Sovereignty,” said in an interview with the Nation, a leftist magazine. She added that the hunger for a wall reflects a deeper anxiety.

“Walls were signifiers for populations in distress, of loss of a national ‘we’ and national control,” she said. “Walls are desired because they mark something: They establish something of an ‘us’ and a ‘them’ and our capacity to take control of situations.”

Even though myriad experts have pointed to arguably more effective 21st-century methods of tightening border security — including deploying ground sensors and drones — Trump clings to the wall as a symbol of his “America First” political commitments and defense of national sovereignty.

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“A wall is so primitive,” Jane Loeffler, an architectural historian, said to The Post’s Marc Fisher. “You can dig under it, go over it, catapult yourself over it. A wall is more symbolic than a real defense. A wall is fear in three dimensions.”

That’s what walls have always been. “There’s always insecurity,” David Frye, the author of “Walls: A History of Civilization in Blood and Brick,” told The Post’s Morning Mix blog. “Insecurity is just a chronic form of fear, and it’s certainly insecurity that has driven the building of both the ancient and the modern walls.”

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Advocates of Trump’s project have gestured to ancient and medieval constructions — from Hadrian’s Wall in Roman Britain to the colossal undertaking of the Great Wall of China — as timeless proof of the efficaciousness of walls. In a garbled appearance before news media last week, Trump himself made the argument: “They say ‘a wall is medieval.’ Well, so is a wheel . . . There are some things that work. You know what? A wheel works and a wall works. Nothing like a wall."

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But historians have been quick to counter, citing the innumerable instances when border fortifications, the curtain walls of empires, and stony ramparts of kings were breached by invaders. “Walls in the actual European Middle Ages simply did not work the way Trump apparently thinks they did,” wrote Virginia Tech’s Matt Gabriele for Post Everything. “If anything, their true function may speak to Trump’s intentions: Poor tools of defense, medieval walls had more to do with reassuring those who lived inside them than with dividing self from other.”

We should not think of historic walls in the same light as Trump’s divisive nationalism. “If we must define the years 500-1500 in a certain slice of Europe as a discrete period and place, it was an era marked by being permeable,” wrote medievalist David Perry for CNN. “Peoples, objects and ideas moved easily throughout the region. There were neither borders to protect nor hordes against which to defend, although plenty of key strategic places were well fortified.”

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The American border, too, was for decades not a place to be fortified, but instead pushed further toward the horizon. In a recent essay, historian Greg Grandin sketched the centrality of the frontier to the American story — a terrain that gave birth to notions of American individualism and ingenuity, no matter the history of “nearly unimaginable terror and grief,” massacres and theft that came with America’s expansion west and south.

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“For over a century, the frontier has served as the defining myth of the nation’s identity, a wide-open threshold into the world and the future,” Grandin wrote.

“Now, rather than the frontier opening up, the border is closing in,” he added. “The nation’s archetype is no longer the pioneer. The icons are now the ICE raider and border agent.”

Trump’s critics worry that his lionization of these security forces prefigures a darker time to come. “Unlike Presidents Clinton and Bush, who conceded defeat when it became clear that their initiatives lacked legislative support, Trump has refused to accept the failure of his border wall project,” wrote political scientists Steven Levitsky and Daniel Ziblatt. They warned that Trump could follow in the path of other leaders who at times used manufactured crises to subvert their democracies and consolidate power.

“In his Oval Office speech on Tuesday, he used the word ‘crisis’ six times in eight minutes,” Levitsky and Ziblatt noted. “That is how autocrats respond to legislative opposition.”