SELLS, Ariz. — An eerie hush settles in at sundown on the Tohono O’odham Nation, which straddles 75 miles of border with Mexico.

The roads crawl with the trucks of Border Patrol agents, who stop unfamiliar vehicles, scrutinize back roads for footprints and hike into the desert wilds to intercept smugglers carrying marijuana on their backs and droves of migrants trying to make it north.

By the bad luck of geography, the only large Indian reservation on the embattled border is caught in the middle, emerging as a major transit point for drugs as well as people.