At the beginning of unit 3 the classical civilizations are gone and religion takes a primary role in defining civilizations. Although the Roman empire never recovers, the eastern half, the Byzantine Empire, continues until the end of this time period and spreads orthodox Christianity and a model of strong centralized government. Buddhism is now a permanent feature of Chinese civilization, and through the dynamic and urban dynasties of the Tang and Song, spreads to Korea, Japan and southeast Asia. Perhaps the most remarkable religious phenomenon of this unit is the rise and spread of Islam. Islamic empires spread culture and ideas, and form spheres of increased trade and contact. The Silk Road trade routes--so prominent in connecting the classical civilizations of the previous unit--are eclipsed by Indian Ocean trade. And finally, nomadic people play a important role during this era, the most important of which are the Mongols who build the largest land empire the world has ever seen. By the end of this unit, western Europe and China are each experiencing a renaissance of culture, Russia is recovering from Mongol domination, and the two hemispheres of the globe are about to be introduced by Columbus. 

As you read the Key Concepts, don't just memorize individual facts (although you do need to know them.) It is important that you know how facts serve as examples of the larger concepts under which they are indexed.