[Verginius Rufus]

After he defeated Maxentius in 312 at the battle of the Milvian Bridge, Constantine controlled the western half of the empire, but he was not ruler of the whole empire until 324.

Clodius Albinus is called a usurper because he lost out. Three men were vying for power--Septimius Severus, Clodius Albinus, and Pescennius Niger. Severus formed an alliance with Albinus until he defeated Niger--then he turned against Albinus and defeated him. Severus was a pretty nasty character and probably a lot of Romans would have been happier if Albinus had emerged the winner.

[SunkenCiv]

It’s interesting to me how the career and denouement of Clodius Albinus (and Magnus Maximus a.k.a. Macsen Wledig, and Carausius...) wound up getting conflated with several of the Arthurian legends, including one (perhaps little known) in which Arthur invaded the continent and never came back.

http://en.wikipedia.org/wiki/Septimius_Severus