This is a 1976 study of the organization and tactics of the Seleucid armies from 312 to 129 BC. The first part of the book discusses the numerical strength of the armies, their sources of manpower, the contingents of the regular army, their equipment and historical development, the chain of command, training and discipline. The second part reconstructs the great campaigns in order to examine the Seleucid tactics. The book provides a lesson in Hellenistic and military history and discusses several questions: how did the Hellenistic armies develop after Alexander? What distinguished the Seleucid army as superior to its Hellenistic contemporaries? The answers illuminate the expansion of Hellenism as we learn how the Seleucid army was used as a military, social and cultural instrument to impose the rule of the dynasty over the vast regions of the Empire and how it helped to shape Hellenistic society in the East.