For any writer, retracing ground covered by a classic of history or biography can be daunting. Who today wants to go one-on-one against Boswell or Gibbon? To be sure, the masters made errors that demand correction, and archaeology and archives can provide illuminating new data. But fresh facts are often double-edged: they are as likely to create new uncertainties about the past as they are to resolve old problems. That leaves the modern writer hemming and hawing where his predecessor made magisterial pronouncements.
A case in point is British historian Hugh Thomas. With Conquest: Montezuma, Cortes and the Fall of Old...