In AD 286 the Roman Empire was split into eastern and western empires, each ruled by its own emperor.
The western empire suffered several Gothic invasions and, in AD 455, was sacked by Vandals.
Rome continued to decline after that until AD 476 when the western Roman Empire came to an end.
Historians most often divided Roman history into two major periods, based on the type of government: the Republic (from 509 B.C. to 27 BCE) and the Empire (27 BCE- c. 476 CE).
Second Rome most commonly refers to Constantinople, which was the capital of the Roman Empire from 330 onwards, lasting as the capital for the subsequent Byzantine Empire until its fall in 1453.