This dissertation examines the causes of Roman military success and expansion beyond Italy, focusing on the initial period of expansion from the First Punic War (264 B.C.E.) to the defeat of the Cimbri (101 B.C.E.). I argue that, while the current demographic explanation of Roman success in this period, which focuses on seemingly inexhaustible Roman manpower, has validity, it represents only part of a solution which must also include the Roman ability to efficiently mobilize and organize the resources of Italy for warfare. Through the examination and comparison of battlefield equipment recovered archaeologically and described in literary sources, this project investigates the cost of Roman military matériel. This project also seeks to situate that matériel cost in the context of Rome’s major rivals in this period in order to show that Rome was able to mobilize comparatively more resources for warfare, both generally and on a per-soldier basis, beyond the established advantage of its manpower. This approach is further extended to include naval warfare, particularly during the First Punic War, by combining the recorded figures for the year-to-year fleet strengths of the Roman and Carthaginian fleets with comparative evidence from the classical Athenian navy to produce rough estimates of the total cost of naval operations. The result of these investigations is to show that Rome’s advantage in this period extended beyond manpower to include a superior ability to mobilize a broad range of economic resources. Finally, this project seeks to investigate the sources for this Roman advantage in resource mobilization. It suggests that the ability of the Roman Republic to marshal such vast reserves was due to the translation of the social institution of clientela into a blueprint for the inter-communal system of alliances in Italy, which in turn enabled Rome to efficiently and extensively harness the economic and demographic power of Italy. Rome’s rivals were not able to extract revenue and manpower from their own holdings as efficiently, leading to a decisive Roman military advantage.