On the way to the desert oasis of Quitobaquito Springs, Puerto Blanco Drive parallels the border with Mexico for about 13 miles through Organ Pipe Cactus National Monument. West of Lukeville and the Mexican town of Sonoyta, the border fence, designed as a pedestrian barrier, undulates up a hillside studded with saguaros before ending abruptly. The high mesh fence of oxidized steel then gives way to a low rail-and-post barricade intended to prevent vehicles from crossing into the United States while allowing wildlife to pass through.

Barely 80 degrees, it’s a surprisingly cool day for early May, with broken clouds filtering the sun. A steady breeze blows, refreshing and unlike the blast-furnace-like gusts that often begin by this time in spring. 

The Sonoran Desert is at its most beautiful. The wildflowers of earlier in the season are gone, but blooming paloverdes fill the gaps between the saguaros, recasting miles of the tawny expanses as a garden of yellow. The saguaros are just beginning to bloom, the tips of their arms thick with mixed clusters of buds and...