When I was a college student in San Diego, I worked part-time at the Natural History Museum doing a variety of jobs. I was a zoology major and was once invited to join the herpetologist and his student assistants in the field. We drove over the mountains and dropped into the desert near the border. I was the only brown person in the car, and when a border patrol helicopter whirred overhead, one of the assistants yelled, with mock urgency, “Hide Donna!” Presumably, that’s what they would’ve done had I been undocumented, though that wasn’t the word in use in the 70s.

Later as we walked in the desert, two white men, seemingly out of nowhere, emerged from a path. “Hey, Maria,” one of them called out to me (because, you know, we’re all named Maria), “you legal?”

Such is the racism born of the border.

That afternoon, we happened upon a sidewinder sidewinding its way toward Mexico. We watched it speed away from us, the triple curve of its body swishing telltale tracks in the sand.

In The Line Becomes a River: Dispatches from the Border, Francisco Cantú describes guiding a snake to a gap in the pedestrian fence so it can make its way into Mexico. This act of accommodation comes after Cantú picks up a migrant named de la Vega from the hospital where he had been treated for kidney failure and drives him to the border patrol station to be processed for deportation.

De la Vega is released from the hospital shirtless, the way he had been found after wandering for six days in the desert. Cantú strips off his own shirt and gives it to him, a humanitarian gesture that goes only so far against the pitiless deportation system.

The migrant who is deported. The snake that is gently nudged through a physical barrier that divides its natural territory. The juxtaposition is a theme that runs through the book: the harsh borderland desert that is habitat to animals and gauntlet to humans.

In these scenes, Cantú is in his first year as a border agent, a job he seeks after academic studies of border policies and politics have rendered the subject too remote and abstract for a true understanding — despite his having grown up near the border, despite the concerns of his mother, who tells him, “The border is in our blood.”

“I don’t know if the border is a place for me to understand myself, but I know there’s something here I can’t look away from,” Cantú writes.

Cantú can’t look away from de la Vega’s naked torso, or the feet of a woman whose blisters he washes and salves. But he is able to accept the abandonment of a dead man on the side of the road by a colleague because transport wasn’t available until the next day.

“We stood for a few more minutes talking about the storm and the human body that lay there in the desert, in the dark and in the rain, and we talked of the animals that might come in the night and of the humidity and the deadly heat that would come with the morning. We talked, and then we went home.”

Such dissociation is what his mother warns about and what he himself comes to fear. But as long as he’s in uniform, he must abide by it. While at the firing range one day, he shoots a small bird perched upon the target to prove to himself that he can take a life. He picks up the bird and holds it in his hands. He digs a hole and buries it. He covers the mound with small stones.

He’s trying to get good at his job, he tells his mother. He’ll figure out what it means later, he insists. But his dreams are trying to tell him what it means now. A wolf haunts his sleep with the threat of impending violence. He is grinding his teeth to bits. He is anxious from lack of sleep. After one particularly violent dream, he realizes he must make peace with the wolf, and he addresses him as “brother.”

Later, no longer a field agent but working in intelligence at a desk job, he sees a falcon on one of the camera feeds. The falcon’s unblinking stare probes Cantú’s conscience: “What cowardice has caused you to retreat from the desert? Why not return to the border’s smoldering edges, why not inhabit the quiet chaos churning in your mind?”

After four years, Cantú leaves the border patrol. He takes a job in a coffee shop while he studies writing to make sense of the things he’s seen and done. José, the maintenance man with whom Cantú talks and shares food each day, says, “He visto muchas cosas.” As if to say, my story counts for something, too.

José’s story assumes the spotlight in the last part of the book, when Cantú comes up against the very system he once worked for trying to help his friend, whom he calls brother, to stay in this country. It’s when José speaks that we understand why, despite the law, despite the border patrol, despite the desert, people cross the border again and again.

Cantú has faced backlash on Twitter and at some of his public events for his stint as a border agent. Could he have interrogated the institution and the violence it engenders without becoming part of it? Could he have articulated the complexities without having worn the uniform?

I don’t believe he thinks he could’ve. There’s a deep and tortured honesty in his writing that comes not just from having the border in his blood, but also from introducing the border patrol into his psyche: “It’s like something inside of me still belongs to it. I’m still part of this thing that crushes.”

But Cantú also crushes something. With José’s story, he thwarts the racialized stereotype that has been used to dehumanize migrants and immigrants. And with this book, he reminds us that the border, which as yet is not a wall, is in some places an imaginary line in the middle of a river. That the border is not just a physical structure. The border is in the blood of millions of people — like Cantú, and like me.