[6] From these relatively obscure provincial origins, Postumus would have risen through the ranks of the army until he held command of the Roman forces "... among the Celts".

[12] Amid the chaos of an invasion by the Alamanni and Franks, and spurred on by news of the defeat and capture of Valerian,[11] the army in Gaul revolted and proclaimed Postumus emperor.

[13][14] The trigger was their defeat in 260[note 1] of a Juthungian army which was returning from Italy laden with prisoners, even though they had been repulsed by Gallienus at Mediolanum (Milan).

[9] Saloninus, on the advice of his praetorian prefect Silvanus (who had coordinated Roman policy in Gaul alongside Postumus), demanded the transfer of the recovered booty to his residence at Colonia Claudia Ara Agrippinensium (Cologne).

[16] He established his capital in northern Gaul, probably at Colonia Claudia Ara Agrippinensium[15] or Augusta Treverorum (Trier),[17] and furnished his government with many of the traditional Roman legislative and executive structures.

[24] Postumus fought successful campaigns against the Franks and Alemanni in 262 and 263; following his victory over them in 263, his coin-types celebrated peaceful themes such as Felicitas Augusti for some time.

[26] Scholars continue to debate whether Postumus originally intended to dislodge Gallienus from Rome or was content to rule only the western provinces.

[15] Perhaps he hoped to achieve some official recognition from Gallienus; what is clear, however, is that Postumus was not overtly separatist and did not revive the 1st-century dream of an independent Galliarum imperium.

[27] For four years Gallienus was too distracted by Germanic invasions and other usurpers in the east to address the situation to his north and west,[28] but in 265 he launched a campaign to defeat Postumus.

[28][29] His first attempt failed when Postumus managed to escape from a precarious situation due to the carelessness of Gallienus's cavalry commander Aureolus.

[12] By the end of 265, Postumus's coin issues were triumphantly commemorating the victory over Gallienus, and the festivities celebrating his quinquennalia continued into the following year.

[12] Laelianus, one of Postumus's top military leaders and the governor of Germania Superior, was declared emperor in Mogontiacum (now Mainz) by the local garrison and surrounding troops (Legio XXII Primigenia).

Marius held sway for a short while before being overthrown by Victorinus, Postumus's erstwhile colleague in the consulship and tribune of the praetorian guard.