The Roman Empire began with the fall of the Roman Republic in 27 BC and finished with the abdication of Romulus Augustulus, the final Roman Emperor, in 476 AD.
Civil war engulfed the Roman Republic in the middle of the 1st c. BC, first between Julius Caesar and Pompey, and finally between Octavian and Mark Antony. Antony was defeated at the Battle of Actium in 31 BC. In 27 BC, the Senate and people of Rome made Octavian Emperor in all but name and gave him the name Augustus, thus beginning the Principate. The success of Augustus in establishing principles of dynastic succession was limited by a number of talented potential heirs dying before him: Marcellus, Gaius and Lucius Caesar to name a few.
The Julio-Claudian dynasty lasted for four more Emperors: Tiberius, (Gaius) Caligula, Claudius, and Nero, before Nero committed suicide and ushered in the Year of Four Emperors in 69 AD, from which Vespasian emerged victorious. Vespasian became the founder of the Flavian dynasty and was followed by his sons Titus and Domitian. Domitian would prove to be unpopular with the ruing classes and would eventually be murdered in a palace plot.
The Flavians were followed by the Nerva–Antonine dynasty, which produced the “Five Good Emperors”: Nerva, Trajan, Hadrian, Antoninus Pius and Marcus Aurelius. In the view of the Greek historian Dio Cassius, a contemporary observer, the accession of the emperor Commodus in 180 AD marked the descent “from a kingdom of gold to one of rust and iron”, a famous comment which has led some historians, notably Edward Gibbon, to take Commodus’ reign as the beginning of the decline of the Roman Empire.
Commodus was also murdered in a palace plot, which led to another civil war and the Year of the Five Emperors. Septimius Severus came out on top and founded the Severan Dynasty, which was tumultuous; an emperor’s reign was ended routinely by his murder or execution, and following its collapse, the Roman Empire was engulfed by the Crisis of the Third Century, a period of invasions, civil strife, economic disorder, and plague.
Diocletian brought the Roman Empire back from the brink, and divided the empire into four regions, each ruled by a separate Emperor, which was called the Tetrarchy. Confident that he fixed the disorders plaguing Rome, he abdicated along with his co-emperor, and the Tetrarchy soon collapsed. Order was eventually restored by Constantine, who became the first emperor to convert to Christianity, and who established Constantinople as his new capital. During the decades of the Constantinian and Valentinian dynasties, the Empire was divided along an east–west axis, with dual power centers in Constantinople and Rome. The reign of Julian, who attempted to restore Classical Roman religion, only briefly interrupted the succession of Christian emperors. Theodosius I, the last emperor to rule over both East and West, died in 395 AD after making Christianity the official religion of the Empire.
The Western Roman Empire began to disintegrate in the early 5th c. AD as Germanic migrations and invasions overwhelmed the capacity of the Roman Empire fight off the invaders. The Roman Empire fell in 476 AD, when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer.