EspaÃ±ol FranÃ§ais PortuguÃªs ÎµÎ»Î»Î·Î½Î¹ÎºÎ¬ Ø§ÙØ¹Ø±Ø¨ÙØ©. The Roman Empire began to slowly decline in the 3rd century AD, one of the main causes of Rome's early decline was a series of plagues, most notably the Plague of Cyprian, which decimated the population of the Empire, making it harder for Roman Emperors to levy armies and raise taxes.