On Election Day, 2018, residents of Nogales, Arizona, began to notice a single row of coiled razor wire growing across the top of the city’s border wall. The barrier has been a stark feature of the town’s urban landscape for more than twenty years, rolling up and over hilltops as it cleaves the American town from its larger, Mexican counterpart. But, in the weeks and months that followed, additional coils were gradually installed along the length of the fence by active-duty troops sent to the border by President Trump, giving residents the sense that they were living inside an occupied city. By February, concertina wire covered the wall from top to bottom, and the Nogales City Council passeda unanimous resolution calling for its removal. Such wire has only one purpose, the resolution declared—to harm or to kill. It is something “only found in a war, prison, or battle setting.”

Living in Tucson, barely an hour north of the border, I have become familiar with both sides of Nogales, crossing over the border to shop, attend meetings, take gifts or supplies to deported friends, or volunteer at a soup kitchen for migrants. In December, as I walked through the pedestrian crossing, I passed by uniformed soldiers transporting long ladders to one side of the port of entry, but I barely registered their significance. The militarization of the borderlands has become so commonplace that one often grows numb to its manifestations. It can seem distant until it reaches out to touch you. Only months later, as I watched images of the concertina wire proliferating on my social-media feeds, did I finally understand what those ladders had been for.

In “The End of the Myth: From the Frontier to the Border Wall in the Mind of America” (Metropolitan), the historian Greg Grandin argues that America’s urge to wall off its borders marks the death of our most potent myth—the galvanizing vision of men and women seeking freedom along a vast frontier, a space for reinvention, unburdened by society, history, and one’s own past. Since the very inception of our country, he writes, the presence of a frontier has “allowed the United States to avoid a true reckoning with its social problems, such as economic inequality, racism, crime and punishment, and violence.” The ever-shifting and expanding frontier also acted as a physical barrier against invasion; as a national-security buffer against foreign enemies, Native Americans, and Mexicans; and as a tenuous escape valve for freed slaves, European migrants, and discontented laborers from crowded Eastern cities.