Lord Byron described Greece as great, fallen, and immortal, a characterization more apt than he knew. Through most of its long history, Greece was poor. But in the classical era, Greece was densely populated and highly urbanized. Many surprisingly healthy Greeks lived in remarkably big houses and worked for high wages at specialized occupations. Middle-class spending drove sustained economic growth and classical wealth produced a stunning cultural efflorescence lasting hundreds of years.

Why did Greece reach such heights in the classical period--and why only then? And how, after "the...