While the adjacent civilizations of the Greeks and the Persians collided, two other empires were taking shape — the Roman Empire on the far western side of Eurasia and China’s imperial state on the far eastern end. They flourished at roughly the same time (200 B.C.E.–200 C.E.); they occupied a similar area (about 1.5 million square miles); and they encompassed populations of a similar size (50 to 60 million). They were the giant empires of their time, shaping the lives of close to half of the world’s population. Unlike the Greeks and the Persians, the Romans and the Chinese were only dimly aware of each other and had almost no direct contact. Historians, however, have seen them as fascinating variations on an imperial theme and have long explored their similarities and differences.