The border’s messages always carry with them hints of violence. In the 19th century, the American frontier was a place of conquest, a place where laws did not apply and deadly clashes could happen at any moment. That aura of risk and brutality still hangs over airports, the closest thing we have to frontier outposts and the gateways to cross-border travel. When travelers step into the secure area of an airport, they leave behind bottles of water, take off their shoes and expose their bodies to X-rays, all for the sake of protecting themselves from the potential violence of terrorists. But this system can perpetrate violence by itself. This month, a ticketed passenger who refused to give up his seat on a United Airlines flight from O’Hare International Airport in Chicago was brutally dragged away by the police. David Dao’s injuries, according to his lawyer, Thomas Demetrio, include a concussion, a broken nose, two lost teeth and sinus damage that could require surgery. Demetrio went on to ask, “Are we going to just continue to be treated like cattle?”

This dehumanization is a common feature of the border. Some years ago, returning home from a holiday in Morocco, my husband and I passed through immigration at Kennedy Airport. The border agent glanced at my passport, which lists Morocco as my place of birth. Then she looked at my husband’s and, with a chuckle, asked him how many camels he had traded for me. Even in my shock, I understood that what the agent was trying to assert was her own authority, her superiority over me. If I had dared to challenge her, I might have ended up subject to a secondary search and further questioning. My silence was the price that the border demanded.

Border walls are literal expressions of our worst fears. Terrorists, rapists, drug dealers and various “bad hombres” are all said to come from somewhere else; drawing lines, we are told, will keep us safe from them. But the lines keep multiplying. What formally counts as the border, according to the United States government, is not just the lines separating the United States from Canada and Mexico, but any American territory within 100 miles of the country’s perimeter, whether along land borders, ocean coasts or Great Lakes shores. That 100-mile strip of land encompasses almost entirely the states of Connecticut, Delaware, Florida, Hawaii, Maine, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Rhode Island and Vermont — along with the most populated parts of many others, including California and Illinois. In total, the 100-mile-wide border zone is home to two-thirds of the nation’s population.

This is such a staggering fact that it bears repeating: The vast majority of Americans, roughly 200 million, are effectively living in the border zone. Any of these people could one day face checkpoints like the one I went through in Sierra Blanca, Tex. They can be asked about their citizenship and, if they fail to persuade the agent — because of how they look, act or sound — they can be detained. The Justice Department established these regulations in 1953 and, though they periodically attract attention, they have never been changed. As we move to erect and enforce more borders, this is another message worth apprehending: Borders do not simply keep others out. They also wall us in.