The rise of Rome from small city-state to world empire is probably one of the best-known stories of military history, at least in broad outline, and Rome remains an archetype of both military efficiency and imperialism in the modern world. At that archetypal level, Rome’s rise can seem both inexorable and inevitable. But it was neither, nor was it an unproblematically triumphal march. Rome’s growth as a state and empire entailed significant internal changes both socially and politically, changes that, in turn, had implications for its patterns of military organization and activity. Unlike the Greeks, whose communal infantry was eventually swallowed and appropriated by large states external to Greece, Rome assumed the role of swallower, if not appropriator. But the communal infantry of the early Roman Republic disappeared with the state and society that spawned it, replaced from within by a professional army serving an autocratic empire. Thus, though it took a different route, Rome ended up at roughly the same destination: the sort of large-state military-political formation pioneered by the Assyrians and Qin. The latter’s successor, the Han, joined with Rome to bookend Eurasia in the Age of Empires.
Yet the different route still mattered. Ideologically and culturally, Rome as an empire bore the stamp of its Republican origins, and if Rome, Han China, and Mauryan India shared deep structural similarities, they differed significantly in ideology and culture, as well as the relationship of their military elites to the state. Part of that difference goes to the very origins of Rome, for—unlike China, India, southwest Asia, and even Greece—Rome had no native chariot warrior elite in its past, and its collective mythic history, based in reality, begins with the expulsion of kings, not their ascent to power.