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

Chapter XIII: Reign Of Diocletian And His Three Associates

The Reign Of Diocletian And His Three Associates, Maximian, Galerius, And Constantius.—General Reestablishment Of Order And Tranquillity.—The Persian War, Victory, And Triumph.—The New Form Of Administration.—Abdication And Retirement Of Diocletian And Maximian.