Modern scholars have refined the discussion of combat motivations by considering ‘unit cohesion’ as an umbrella term for a complicated and debated set of forces, which are exerted within military groups, small and large. In the United States, research into the effects of unit cohesion led to the creation of the COHORT system in 1981. This established the concept of combat units ‘based around a cohesive nucleus of soldiers’. A recent meta-study argued that there is no evidence for a causal relationship between social cohesion and effectiveness in combat. This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book examines the text of Aeneas Tacticus and how unit cohesion is represented in it. It discusses aspects of unit cohesion in Roman armies.