Galerius started as an ordinary soldier in the armies of Aurelian and then Probus. By the time he served under Diocletian his military career had culminated with the position of Praetorian Prefect. Under Diocletian's new scheme for ruling the empire, he named Galerius as one of the Caesars in the new Tetrarchy and assigned him to the eastern half. With Diocletian abdicating soon after, Galerius automatically became Augustus himself.
The rest of his reign would be taken up fighting the power grabbing of Constantius Chlorus who, against the principles of the Tetrarchy, would start a dynasty in his own bloodline as well as dealing with the increasingly troublesome Maxentius who was now leading a revolt from Rome with the aim of eliminating what was left of the Tetrarchy. He died of natural causes before any conclusions were in store for the stalemate.