The border wall is about exclusion, not security. (Gregory Bull/AP)
Elizabeth Greenspan is a senior researcher at PennPraxis at the School of Design at the University of Pennsylvania and the author of Battle for Ground Zero: Inside the Political Struggle to Rebuild the World Trade Center.

A few months before the first anniversary of 9/11, a conflict erupted at Ground Zero about what kind of barrier should surround the 16-acre site. Recovery workers had mostly completed their search for victims’ remains, and cleanup crews were making rapid progress clearing rubble. Meanwhile, the city’s system of ad hoc plywood walls and chain-link fencing, hastily erected after the twin towers collapsed, was starting to become ragged. It was time to build a more permanent barrier.

The site’s owner, the Port Authority of New York and New Jersey, announced that it was going to build a 40-foot tall wall to stand for five to 10 years as it rebuilt. Such a wall was necessary to “protect” people, it said, from seeing “disconcerting views” as they walked through Lower Manhattan. Within hours of the announcement, however, a resistance mobilized. Many interpreted the wall as a real and symbolic means of exclusion. The World Trade Center’s site’s future was still up for grabs, and many in the city saw the wall as a move by the site’s owner to exert control and keep the public out of decision-making.

Watching President Trump try to build a wall on the U.S.-Mexico border has conjured up memories of this earlier conflict. The WTC site and the border are very different places, but a desire to claim ownership drove both proposals to replace battered fencing with a wall. People and institutions turn to walls particularly when they fear they’re losing control of a place they think should belong exclusively to them. Fences mark boundaries and keep people out, but only walls, by concealing completely, tell people they’re not welcome.

In New York, the struggle for ownership was quite literal. The Port Authority and developer Larry Silverstein owned the legal right to rebuild, but many New Yorkers demanded a say in determining what would stand there. After all, the site had deep symbolic meaning as the location of the deadliest foreign attack in the country’s history, claiming nearly 3,000 lives. At scores of public hearings and town halls, thousands of people directly challenged the owners’ authority over the land.

Immediately after the Port Authority’s wall announcement, a group of local architects spent nights and weekends designing a counter-barrier: a fence, 12 feet tall, made of stainless-steel mesh so people could see through it. The design contained small ledges for people to leave memorials and boards to write messages. One of the architects told me that where the wall concealed and excluded, the fence expressed “transparency and openness.” Simply seeing the site facilitated a kind of public participation and access, she said, as did leaving notes, flowers and homemade shrines.

The struggle over the boundary was understandable, given so many people’s strong feelings about Ground Zero. But it felt surreal, too, as if people thought the barrier itself would determine the site’s future. Of course, it wouldn’t. The most important rebuilding decisions were being made behind closed doors, among lawyers, politicians and owners. Even though everyone seemed to know this, they dived in anyway, seduced by the idea that the decision — wall or fence — would have consequences for determining control of the site. It was an architectural proxy war.

After months of negotiations, the Port Authority announced a new plan. It would give the people their fence, but it enacted special rules that protected its ownership, including barring homemade memorials.

A consultant to the Port Authority explained the prohibition. “Giving people a place to leave a teddy bear, that would make [the fence] permanent,” he said. The Port Authority feared that when it came time to make way for new buildings, people would resist the removal of a memorial-filled fence, complicating the rebuilding effort. It sounded a little far-fetched, but it spoke directly to the Port Authority’s broader fear: losing authority over the site to an empowered public.

At the U.S.-Mexico border, people are fighting another sort of proxy war through walls and fences, one over the country’s figurative ownership of the dividing line between the two countries. Trump proposed a wall with xenophobic nods to protection and safety. But millions of people immediately understood the proposed wall as an expression of his supporters’ desire to assert dominance and defend against lost status and power. As Trump likes to say, he’s “taking our country back.”

Yet after Trump won, a lot of people — including Republican politicians, Border Patrol agents and many who live near the border — began waging a campaign for fencing instead of a wall. It turns out that fences are safer than walls because they allow guards to see the other side. As an anonymous border guard told CNN in February, “I’m not calling it a wall because we are talking about a fence that we can look through. That’s what we need.”

But because a fence doesn’t express the exclusion and sense of ownership that many of Trump’s supporters want, Trump will only call it a wall. Speaking to reporters in July, Trump said, “One of the things with the wall is you need transparency.” He added, “You have to be able to see through it.” Expect to hear a lot about “see-through walls.”

Debates about walls and fences are often only partially about actual walls and fences. Instead, they’re flash points in broader political debates about who owns a piece of land, or even an entire country, and what kind of place it should become.

The outcome of the wall/fence battle at Ground Zero didn’t appear to have great effect on ownership or control. In the end, the Port Authority and Silverstein built largely what they wanted. But the conflict was revealing as a microcosm of the broader rebuilding effort. In a way, it mirrored the owners’ approach to subsequent fights and messes (of which there were many): making small, carefully considered compromises that never altered the ownership calculus or undermined the rebuilding agenda. In fact, the owners often used compromises to advance their agenda, thus helping maintain control.

We don’t yet know what will happen at the border. But the result, whether it’s a wall, a fence or nothing at all, will probably be less important than Trump’s approach to getting it done — and then, what Americans make of it. Trump has understood from the very beginning that a wall serves as a useful proxy for the larger conflicts his supporters want him to fight. This summer, he has crusaded for the wall at every opportunity — even threatening to shut down the government if Congress doesn’t deliver billions to fund its construction. He’s banking on the wall, and all that it symbolizes, getting him through some tough times.