The three centuries which followed the conquests of Alexander are perhaps the most thrilling of all periods of ancient history. This was an age of cultural globalization: in the third century BC, a single language carried you from the Rhone to the Indus. A Celt from the lower Danube could serve in the mercenary army of a Macedonian king ruling in Egypt, and a Greek philosopher from Cyprus could compare the religions of the Brahmins and the Jews on the basis of first-hand knowledge of both. Kings from Sicily to Tajikistan struggled to meet the challenges of ruling multi-ethnic states, and...