This companion provides an extensive account of the Roman army, exploring its role in Roman politics and society as well as the reasons for its effectiveness as a fighting force. * An extensive account of the Roman army, from its beginnings to its transformation in the later Roman Empire * Examines the army as a military machine - its recruitment, training, organization, tactics and weaponry * Explores the relationship of the army to Roman politics, economics and society more broadly * Considers the geography and climate of the lands in which the Romans fought * Each chapter is written by a leading expert in a particular subfield and takes account of the latest scholarly and archaeological research in that area