Europe 293: First Tetrarchy
By 293 Carausius still held out in Britain and Diocletian had become convinced that a new system was needed to secure both the Roman Empire and the succession. In March he presided over the appointment of two Caesars (subordinates and heirs)—Galerius to serve with him in the East and Constantius to join Maximian in the West. This rule of four men—two Augusti (emperors) and two Caesars—later became known as the Tetrarchy.
The Agri Decumates was lost to the Romans in c.262, regained by Aurelian and Probus in 275–8, and lost again sometime between 290 and 310. The losses here seem not so much due to any rise in power of the local Alemanni tribe, but Roman internal division. When rival Roman factions controlled Gaul and Raetia, as was the case in 262–274 and 306, the limes of the Agri Decumates were no longer defensible and had to be abandoned.
Capitals of the First Tetrarchy
Officially, Rome remained the capital of the Roman Empire throughout the Tetrarchy. In practice, the real capital was wherever Diocletian’s court—and to a lesser degree, that of the other tetrarchs—resided. This varied considerably in the East, where Diocletian was initially based at Sirmium, in the Balkans, then joined Galerius in Antioch, before finally swapping places with his Caesar altogether by taking command at Antioch and Nicomedia while Galerius took charge in the Balkans (at Thessalonica and then Serdica). In the West the situation was less fluid: except for providing some brief support on the Rhine and a few years spent in Spain and Africa, Maximian remained based at Mediolanum, in Italy; his Caesar, Constantius, was largely based at Augusta Treverorum, in Gaul.
287 Diocletian–Bahram II Treaty▲
In 287 Diocletian moved against Sasanian Persia, but a major war appears to have been averted when Shah Bahram II agreed to a treaty with the Romans. The terms of the agreement are unknown, but Armenian tradition, supported by Roman panegyrics, holds that Diocletian installed Tiridates III as king in part of Armenia at this time. Tiridates would later embrace Christianity and make Armenia the first officially Christian state (both in 301).
289 Diocletian’s Second Sarmatian Campaign▲
By 289, three years after their earlier incursion, the Sarmatians crossed the Danube into the Roman Empire again. Diocletian defeated them a second time, but established his main residence at nearby Sirmium in preparation for further problems.
289–290 Maximian vs Carausius▲
Throughout 288 Maximian had men at work constructing warships along the rivers of Gaul for the invasion of Britain and the overthrow of Carausius. The fleet was ready the following year, but, after Maximian successfully seized control of the Channel coast of Gaul, a storm hit and destroyed his ships. Carausius then seems to have exploited the situation to regain control of Gesoriacum (Boulogne), prompting the luckless Maximian to agree to peace in 290.
? ?? 289–10 Mar 298 Second Quinquegentiani War▲
In 289 the Quinquegentiani (“five peoples”), a confederation of Berber tribes in the highlands of Kabylia, launched a renewed rebellion against Roman rule in North Africa. Eventually, in 296, Maximian travelled to Africa to deal with the rebels and in the following years drove them back into the Atlas Mountains. By March 298, when Maximian celebrated his triumph in Carthage, the Romans had pushed the Quinquegentiani into the Sahara, after which they disappeared from history.
Between the end of Maximian’s Rhenish War (286–288) and mid-291, the Burgundi turned on their erstwhile Germanic allies, the Alemanni, and seized large amounts of territory from them. The war was costly for both sides and the embittered Alemanni requested, but did not receive, Roman support to recover the lost lands.
In 291, or possibly 290, a group of Goths called the Tervingi aligned with another new group, the Taifali, to attack the Hasdingi Vandals and Gepids in eastern Dacia. It is generally believed that the Tervingi won this conflict as by the fourth century they were the foremost power north of the Danube.
1 Mar 293 First Tetrarchy▲
In order to further secure both the Roman Empire and the succession, Diocletian presided over the appointment of two Caesars—Constantius to serve with Maximian in the West and Galerius to join Diocletian in the East. Both men, who had already proved themselves as loyal officers, divorced their current wives to marry the daughters of their superiors. For reasons unknown, Constantius was also given the title Jovius (Jupiter) and Galerius the inferior title Herculius (Hercules). This rule of four men—two Augusti and two Caesars—later became known as the Tetrarchy.