Exit 132 off Interstate 29 in Brookings, South Dakota, offers two possibilities. A right turn will take drivers through miles of farms, flatland that stretches to the horizon, cut up into grids by country roads and picturesque barns—a scenic route to nowhere in heartland America. But take a left at the light, and you wind up coasting through a college town of 19,000 that’s more than 95 percent white. The city’s small Latino minority—less than 1 percent of the population—is mostly students or faculty members passing through South Dakota State University. It was here, in late 2009, that I experienced an epiphany about Mexican food in the United States. 

I had been visiting the campus and found myself desperate for a taste of home. For us Southern Californians, that means burritos. Google Maps found me four Mexican restaurants in town. One, named Guadalajara, is a small South Dakota chain with outposts in Pierre and Spearfish. The food there was fine: a mishmash of tacos, burritos, and bean-and-rice pairings. But talk to the waiters in Spanish, and their faces brighten; they trot out the secret salsa they make for themselves but don’t dare share with locals for fear of torching their tongues. 

The most popular restaurant in town that day was Taco John’s. I didn’t know it then, but Taco John’s is the third-largest taco chain in the United States, with nearly 500 locations. But what lured me that morning was a drive-through line snaking out from the faux-Spanish revival building (whitewashed adobe and all) and into the street. Once I inched my rental car next to the menu, I was offered an even more outrageous simulacrum of the American Southwest: tater tots, that most Midwestern of snacks, renamed “Potato Olés” and stuffed into a breakfast burrito, nacho cheese sauce slowly oozing out from the bottom of the flour tortilla.

There is nothing remotely Mexican about......