The reign of Marcus Aurelius had been marked by almost continuous warfare, even though he preferred books to war. That of Commodus was comparatively peaceful in the military sense but was marked by political strife, and the arbitrary and capricious behaviour of the emperor himself. In the view of Dio Cassius, a contemporary observer, his accession marked the descent "from a kingdom of gold to one of rust and iron". The historian Edward Gibbon took Commodus's reign as the beginning of the decline of the Roman Empire.