Following his African conquests, the Roman Empire may have reached its greatest extent during the reign of Septimius Severus, David L. Kennedy, Derrick Riley (2012), Rome's Desert Frontiers, page 13, Routledge R.J. van der Spek, Lukas De Blois (2008), An Introduction to the Ancient World, page 272, Routledge under whom the empire encompassed an area of 2 million square miles ( 2 sqmi million square kilometers).