 Complete the following phrase:  Since the age of humanism, the process of the Fall of the Western Roman Empire in which it failed to enforce its rule, has been thought to have begun with Constantine the Great, or with the soldier emperors who seized power through command of the army from 235 through 284, or with Commodus, or even with Augustus. The loss of centralized political control over the West, and the lessened power of the East, are universally agreed, but the theme of decline has been taken to cover a much wider time span than the hundred years from 376. For Cassius Dio, the accession of the emperor Commodus in 180 CE marked the descent from a kingdom of gold to one of rust and iron. Gibbon also began his narrative of decline from the reign of Commodus, after a number of introductory chapters. Arnold J. Toynbee and James Burke argue that the entire Imperial era was one of steady decay of institutions founded in republican times. Theodor Mommsen excluded the imperial period from his Nobel Prize-winning History of Rome 185456. As one convenient marker for the end, 476 has been used since Gibbon, but other key dates for the fall of the Roman Empire in the West include the Crisis of the Third Century, the Crossing of the Rhine in 406 or 405, the 