It was the ambition of every man who took the throne to reenact Augustus’ creation of a hereditary monarchy. Augustus and Livia, however, had had to rely on adoption when they failed to produce a male heir to the throne; likewise, since subsequent Roman emperors would remain formally monogamous, the chances of producing a biological heir who would survive to reach the throne were slim. Vespasian was the only emperor in the first century to be succeeded by biological children, something that would not happen again until Commodus succeeded Marcus Aurelius in 180. In the next century, Septimius Severus and Valerian - although by no means the only ones to have sons - were the only rulers to be succeeded by male children until Constantius I engineered the accession of his son, Constantine, in 306.
Adoption was a surer path to dynastic longevity than biology, for it had the coincidental benefit of ensuring that a competent adult would be in line for succession, while, at the same time, giving the reigning emperor room to negotiate with the governing class (for details, see Peachin, this volume). It is perhaps an accident of fortune that many of the men who took the throne between 235 and 253 either had male children, often rather young, or brothers. Shackled by the fact of their biological success, emperors lacked the ability to negotiate the succession within the governing class. The presence of male children may be seen as one of the genuinely unusual features of third-century history; one of the keys to Diocletian’s success in breaking the cycle of instability and violence that had troubled the empire in the half century prior to his accession now becomes clear: Diocletian had a daughter.
The situation with respect to the frontiers was more complex. The sudden failure of the Roman state to hold the initiative in foreign affairs may be attributed above all else to stagnant military doctrine and related failures of imagination (Potter 2004: 125, 213). The tactical organization of the Roman army in 235 had not changed significantly since the end of the second century bce (Pollard, this volume). The long wars waged by Marcus Aurelius against northern tribes should have served as a wake-up call since the fact that the emperor was forced to spend most of his reign on the frontiers suggests that the tribes had adapted themselves to the Roman style of war. So too should the successful Parthian invasion of Commagene and Syria in 161 have raised some question as to the fitness of the army. Although the Parthians were repulsed, and their capital at Ctesiphon laid waste in 165, the incursion of 161 was the most dangerous assault upon the Roman east since the forties bce. Roman success in the war may be attributed to the ability to force the Parthians to defend fixed points, and the weakness of the Parthian military establishment. The emergence of the vastly more competent Sasanian regime in 225 ce changed the balance of power in the east just as tribes north of the frontier developed new ways of penetrating static Roman defensive lines along the northern borders.