27 BC to 476 AD or 1453 AD (depending on your perspective)
27 BC - Octavian "Caesar Augustus" becomes the sole Triumvir after the Battle of Actium. He is the "Imperator", or emperor.
382 AD - Emperor in the Western Roman relinquishes the title of Pontifex Maximus. The Bishop of Rome (or Pope) assumes this title for himself.
395 AD - The eastern and western portions of the Roman Empire are permanently split into two separate realms
476 AD - The Western Roman Emperor in Italy abdicates. There remains an emperor in the Eastern Empire. In addition, Dalmatia and Soissons remain under the control of isolated Roman officials. The Pope remains as a traditional Roman official in the west.
480 AD - Roman Dalmatia falls to King Odoacer of Italy
486 AD - Roman Soissons falls to the Franks
534 AD - Eastern Roman Empire re-captures North Africa
536 AD - Eastern Roman Empire retakes Rome and most of Italy
536 AD-1453 AD - Roman empire is eroded by invasions and war to a few small cities in Greece.
1453 AD - Constantinople, the last Roman city, falls to the Ottomans. The last Roman Emperor dies defending the city.