History of the Roman Empire disputes about the supreme authority, this finally formalised a peaceful succession of the emperor: in each half a "Caesar" would rise up to replace the "Augustus" and select a new "Caesar". On May 1, 305, Diocletian and Maximian abdicated in favour of their "Caesars". Galerius named the two new "Caesars": his nephew Maximinus for himself, and Flavius Valerius Severus for Constantius. The arrangement worked well under Diocletian and Maximian and shortly thereafter. The internal tensions within the Roman government were less acute than they had been. In "The Decline and Fall of the Roman Empire", Edward Gibbon notes that this
Historiography of the fall of the Western Roman Empire catastrophic: Peter Heather, in his "The Fall of the Roman Empire" (2005), maintains the Roman imperial system with its sometimes violent imperial transitions and problematic communications notwithstanding, was in fairly good shape during the first, second, and part of the 3rd centuries AD. According to Heather, the first real indication of trouble was the emergence in Iran of the Sassanid Persian empire (226–651). As reviewed by one writer on Heather's writing, Heather goes on to state—in the tradition of Gibbon and Bury—that it took the Roman Empire about half a century to cope with the Sassanid threat, which it did