The assassination of Emperor Commodus in 192 sparked a civil war. Septimius Severus emerged as the eventual victor and his dynasty (the Severans) ruled until 235. He fought numerous campaigns, against both internal rivals and external enemies, extending the Empire to the east (adding Mesopotamia), the south (in Africa) and the north (beyond Hadrian's Wall). The military aspects of his reign, including his reforms of the army, are the main focus of this new study.

After discussing his early career and governorship of Pannonia, Michael Sage narrates his war with Pescennius Niger, the siege of Byzantium, and the campaign in northern Mesopotamia that added it as a province. The much more difficult campaign against Clodius Albinus in Gaul is also studied in detail, as is that in North Africa. The narrative concludes with an account of the last campaign in Britain and Severus' death. The final chapters analyse Septimius' reforms of the army and assess their impact on events of the next seventy years until the accession of Diocletian. His greatest weakness was his love for his family. Like Marcus Aurelius he loved his children too much. They failed to maintain what he had bequeathed them.