THE mound lies just beyond the oasis town of Sheberghan in northern Afghanistan, on the plain that slips south to the Hindu Kush and north to the banks of the Amu Darya, or Oxus. This was once Bactria, where the Hellenic world briefly touched and intertwined with the worlds of the Indus and the Siberian steppe. Greeks prospered here for a century or so after the death of Alexander the Great, in 323BC, and then were driven off. The mound is anonymous now, barely noticeable from the road. It stands three metres (ten feet) high, 100 metres in diameter, lopped...