BEFORE ANYTHING ELSE, there was an idea: an imagined demarcation, long contested but still undefined. So, in 1849, a group of surveyors—half Mexican and half American—set out to finally map the line dividing their two nations. It was, at the time, largely an intellectual exercise: careful measurements toward an ideal, and still more plan than physicality, more symbol than safeguard. But still, there was a need for some kind of map. Treaties, after all, had been signed; decisions had been made. It was time, finally, to formalize the space where one sovereignty ended and another began.
Topography was the easier part to understand: the Rio Grande and the Colorado River. Those generous, physical outlines of division. Gifts from nature, when first one country and then the next decided, finally, on…
