Imperial colleges were not unusual when in 293 Diocletian co-opted Galerius as his Caesar and Maximian did the same for Constantius. Many Augusti had made their sons into Caesars, and Diocletian’s regime was not the first to consist of two Augusti. There had also been brief anomalies. Between 166 and 169, Marcus Aurelius and Lucius Verus headed their own college of four, with Commodus and Annius Verus as Caesars. Between 209 and 211, Septimius Severus, Caracalla and Geta ruled as three Augusti. But what made Diocletian’s Tetrarchy unique was the fact that the new Caesars were active rulers like their Augusti. Previously, Caesars had mostly been children with nominal power, but Constantius and Galerius were adults who actively ruled in their parts of the empire. With the expansion of the Dyarchy into the Tetrarchy, there were, in a very real sense, four emperors. Why did Diocletian and Maximian expand the imperial college in such an unprecedented manner? It is proposed that military rebellion, and regional interests and identity more broadly, rather than foreign threats, were major influencing factors in the decision to expand the imperial college and on the way that it functioned. The Tetrarchy was a scheme of imperial presence.

Dating the Appointments

The date(s) on which Constantius and Galerius became Caesar is a matter of debate, and one that has a bearing on whether the Tetrarchy was planned or improvised. Coins, papyri and inscriptions show that there were only two emperors before 293, and Constantius’ panegyrist in 296/7 claims that the Augusti co-opted both Caesars on 1 March, employing the symbolism of the beginning of spring (Pan. Lat. 8[5].2.2–3.1). Lactantius likewise asserts that Galerius’ uicennalia (twentieth anniversary of rule) was to be celebrated on this day (DMP 35.4), and earlier he claims that Galerius’ accession took place at a ceremony outside Nicomedia, on the same spot where Maximinus would be acclaimed Caesar in 305 (19.2). The Descriptio Consulum, a multiauthored list of consuls whose relevant entry was written in the early fourth century, also gives 1 March as the date, despite recording the event under the year 291 (Chron. Min. 1.230).

However, there is reason to think that Galerius was actually co-opted later than Constantius.