Barbed wire and antiaircraft guns ring a maze of buildings in the Iranian desert that lie at the heart of the West’s five-year standoff with Tehran over its program to enrich uranium. It is a place of secrets that Iran loves to boast about, clouding the effort’s real status and making Western analysts all the more eager for solid details and clues. Tehran insists that its plans are peaceful. But Washington and its allies see a looming threat. The sprawling site, known as Natanz, made headlines recently because Iran is testing a new generation of centrifuges there that spin faster...