History seldom turns sharp corners, and historians often have difficulty deciding just when one phase of the human story ends and another begins. Between roughly 200 and 850 C.E., many of the second-wave states and civilizations (Han dynasty China, the western Roman Empire, Gupta India, Meroë, Axum, Maya, Teotihuacán, Moche) experienced severe disruption, decline, or collapse. For many historians, this has marked the end of an era and the start of a new period of world history. Furthermore, almost everyone agrees that the transatlantic voyages of Columbus beginning in 1492 represent yet another new departure in world history. This coupling of the Eastern and Western hemispheres set in motion historical processes that transformed most of the world and signaled the beginning of the modern era.

But how are we to understand the thousand years (roughly 500 to 1500) between the end of the second-wave era and the beginning of modern world history? Frankly, historians have had some difficulty defining a distinct identity for this millennium, a problem reflected in the vague terms used to describe it: a postclassical era, a medieval or “middle” period between the ancient and modern, or, as in this book, an age of third-wave civilizations. At best, these terms indicate where this period falls in the larger time frame of world history, but none of them are very descriptive.