Brian Campbell 
Warfare and Society in Imperial Rome, C. 31 BC-AD 280 [PDF ebook] 

This well-documented study of the Roman army provides a crucial aid to understanding the Roman Empire in economic, social and political terms. Employing numerous examples, Brian Campbell explores the development of the Roman army and the expansion of the Roman Empire from 31 BC-280 AD.When Augustus established a permanent, professional army, this implied a role for the Emperor as a military leader. Warfare and Society in Imperial Rome examines this personal association between army and emperor, and argues that the Emperor’s position as commander remained much the same for the next 200 years.