The Augusti wanted militarily experienced men to serve as their Caesars, and so they co-opted their martial sons-in-law, whom they subsequently adopted. The fact that they co-opted their sons-in-law and adopted them was not inherently unusual. The Julio-Claudian and Nerva-Antonine emperors had repeatedly strengthened their connection to non-filial heirs through adoption. But by 293 Maximian had a biological son: the child Maxentius. Up to this point, only under extraordinary circumstances had emperors passed over their blood descendants when it came to the imperial succession. Augustus rejected his grandson Agrippa Postumus as a potential successor, but this was because he had banished Agrippa as a traitor. Septimius Severus, even though he had two sons, named a rival imperial claimant, the general Clodius Albinus, as his Caesar. However, it is evident that Severus was aiming to postpone a war with Albinus while he dealt with Pescennius Niger, another rival commander. Following his victory over Niger, Severus recognised his son Caracalla as Caesar, thus igniting a war with Albinus. Moreover, from the late first century onwards, emperors only adopted when they lacked a biological son. Before this time, the Julio-Claudian emperors had employed adoption to furnish themselves with dual heirs. Augustus adopted his stepson/son-in-law Tiberius despite having a grandson in Postumus Agrippa. Tiberius adopted his nephew Germanicus while having a son in Drusus, and he later made Germanicus’ son Caligula his joint heir, along with Drusus’ young son Gemellus. Claudius adopted his stepson/son-in-law Nero despite his son Britannicus. However, this practice had lent itself to intra-dynastic murder. Unsurprisingly, it did not outlast the Julio-Claudians.

Dynasties were the norm, and by the third century emperors prioritised their biological sons as heirs. This custom was taken seriously enough that children and teenagers could become emperor. Caracalla was eight or ten when he became Caesar and nine or eleven when he became Augustus. Severus Alexander was Caesar at twelve and Augustus at thirteen. Gordian III was twelve or thirteen when he became Caesar and then Augustus. Philip II was Caesar at five or six and Augustus at eight or nine. In the last chapter, I noted that it made sense for the Augusti to select Constantius and Galerius as their heirs if they wanted their Caesars to be adult military professionals, but this should not obscure the fact that what they had done was unprecedented.