The History of the Decline and Fall of the Roman Empire, by Edward Gibbon

Chapter XIV: Six Emperors At The Same Time, Reunion Of The Empire

Troubles After The Abdication Of Diocletian.—Death Of Constantius.—Elevation Of Constantine And Maxentius.— Six Emperors At The Same Time.—Death Of Maximian And Galerius.—Victories Of Constantine Over Maxentius And Licinus.—Reunion Of The Empire Under The Authority Of Constantine.