For most of his reign, Postumus stayed as a nemesis of Gallienus with the traitor’s help, Aureolus.

Following a dispute with Saloninus, Postumus arrived at the scene with the distribution of the war loot being captured. 

Since, the command of the military was in hands of Postumus, there is little to defend him for Saloninus apart from his official recognition and title.

Having named himself as the emperor, Postumus not only captured but also executed Saloninus, who was a threat to his fate and empire.

Little was done by the rest of the empire at that particular moment. Thus, Gallienus got into conflict with several usurpers on the eastern side, and therefore was unable to avenge the death of his brother.

Rome couldn’t do anything about that, and for that particular time, the western half of the empire was under Postumus, being a de facto ruler.

Finally, after returning from the eastern side, Gallienus found the Gaul being entrenched by Postumus. While also snatching Spain and Britain far away from the empire.

Since, his power grew during the presence of Gallienus, he had a certain time to get his leadership consolidated, while also imposing a big threat.

For one particular reason or another, preparations were never made by Postumus to either mount an offensive against Gallienus or attack Rome. Meanwhile, he got contended himself with the secessionist state.

However, he was aware that all his options were being mulled by Gallienus, and his agenda to recapture the lost lands were held by him.

A number of expeditions were mounted by the Gallienus in order to dispose of Postumus, however, everyone failed. Despite that, Gallienus was the most likely winner, if Postumus hadn’t got help from one of the trusted men of Gallienus. The helper’s name was Aureolus, and he had planted a successful conspiracy that ended with Gallienus’s murder.

A deadly fate had to be met by Postumus himself, in the year following a revolt that was ignited by his leading general (Laelianus). This was a move that is extremely reminiscent of his own power rise.