On the death of Trebonianus Gallus (47; r.251-53), Valerian (c.65; r.253-260) was proclaimed emperor by his troops. It was decided that the Empire, which was having vast problems in both Europe and Asia, would be split in two with Valerian ruling the East, where he faced the Persian threat, and his son Gallienus (c.50; r.253-268) taking control of the West. In 260 when Valerian was captured and imprisoned by the Persians, Regalianus, (260/1) governor of Pannonia Superior, proclaimed himself emperor at Carnuntum (northeast Austria).
Before moving against him, Gallienus (c.50; r.253-268) succeeding his father Valerian, left his son Saloninus (fl.258-260) at the base at Cologne, west Germany to represent the ruling ‘dynasty’. Saloninus, being only a boy, was entrusted to the praetorian prefect Silvanus (fl.260), while the military defence of the Rhine was put in the hands of Gallienus’ general Casianus Latinius Postumus (fl.259-268).
In autumn 260 Silvanus, acting on behalf of Saloninus, challenged Postumus to hand over some loot recaptured from a Frankish raiding party. Postumus, however, insisted on distributing the loot among his victorious troops. When Silvanus persisted, the troops mutinied (by now the news of Valerian’s capture had reached the West) and proclaimed Postumus emperor. Silvanus and Saloninus tried to rally troops at Colonia Agrippina, but it was too late, the garrison murdered the two men and opened the gates to Postumus.
With Gallienus fully occupied elsewhere, Postumus was able to consolidate his position. Gaul, Britain and a large part of Spain joined him and formed the Imperium Galliarum or Gallic Empire (260-274). In 265, Gallienus’ cavalry commander Aureolus (fl.258-268) defeated Postumus and then besieged him at a nearby town (unknown). When a defender shot Gallienus in the back with an arrow, the campaign was abandoned, leaving Postumus the undisputed ruler of the Gallic Empire. In 269 Laelianus rebelled against Postumus, who defeated him then faced another usurper Marius. In the fighting against Marius, Postumus stopped his troops from sacking Mainz, and so the disappointed soldiers killed him.
Postumus was succeeded by Victorinus (r.269-271), who was assassinated and succeeded by Tetricus (r.271-274). In spring of 274 Aurelian (60/2; r.270-275) met and fought Tetricus near Chalons-en-Champagne in France. Tetricus surrendered and within a few months Aurelian had gained control of the Gallic Empire and reunited it with Rome.
In 285 the Roman emperor Diocletian (67; r.284-305; d.312) chose Maximian (c.60; r.286-305) as his caesar (junior colleague and successor) and raised him to co-augustus (co-emperor) the following year: Maximian was to govern the western provinces while Diocletian administered the eastern ones. In 293 Diocletian created the Tetrarchy by adding a caesar to each augusti.
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