Roman Empire was one of the greatest empires of ancient world and the greatest in the ancient European history. Roman Empire was founded in 27 BC when Augustus became Emperor and he replaced the Roman Republic with Roman Empire. In 117 AD, the Roman Empire reached at its zenith when many parts of western and southern Europe, North Arica and western Asia came under the control of the Empire. But due to expansion of the empire in the far flung areas, it became much difficult for the emperors to control the areas away from the center. The Crisis of the Third Century (235-284 AD) made it impossible for the emperor to rule the whole of Roman Empire and the empire disintegrated into the Eastern Roman Empire (Byzantine Empire) and Western Roman Empire in 285 AD.
Roman Empire was under attack from both of its eastern and western sides. Germanic Tribes were their main enemy on the western side while they fought against Parthian Empire (and Sassanid Empire later) of Persia continuously on their eastern side. Despite its efforts, the Roman Empire could not vanish any of its components at its right and left flank and it was becoming more and more difficult for the Empire to keep full attention on both the sides.
Uprisings and rebellions were common in Empire and most of the time the Roman Empire was able to defeat the rebellions completely. But rebellions during wartime were much harder for the Roman Empire to be defeated.
Roman Emperor Alexander Severus was assassinated in 235 AD. This assassination led to a bloody civil war that lasted for 50 years (235-284 AD) and is known as the Crisis of the Third Century. This crisis was the main cause of the First Division of the Roman Empire.
In 285 AD, Roman Emperor Diocletian appointed Maximian as the Emperor of the Western areas of the Empire. Both were aided by two Caesars (subordinates), Galerius and Constantius Chlorus. Practically, there were four emperors of Roman Empire due to which this system is known as Tetrarchy.
This division didn’t last long as Constantine the Great defeated all his opponents and became the sole emperor of the whole Roman Empire in 324 AD.
Constantine died in 337 AD, after which a civil war started among the three sons of Constantine for the control of Roman Empire which resulted in the division of the empire in three parts. Later, Constantius II reunited the Roman Empire in 353 AD and became its Emperor. He is known as the first ruler of Byzantine Empire (Eastern Roman Empire) as he moved his capital to the city of Byzantium (later Constantinople and now Istanbul).
In 364 AD, Valentinian I became the emperor of the Roman Empire who immediately divided the Roman Empire again into eastern and western side by giving the eastern side of the empire to his brother Valens. This division lasted afterwards as no one tried to reunite the whole Roman Empire again.
The Western Roman Empire could not survive longer as it vanished completely in 476 AD after the Battle of Ravenna when Germanic tribes easily defeated the remnants of Roman army and different areas of Italy were divided among various Germanic tribes.
On the contrary, the Eastern Roman or the Byzantine Empire was able to survive for around 1000 more years. In the 7th Century, they lost their Arab and North African parts to the invading Muslim Arab armies but their capital Constantinople was occupied by the Ottomans in 1453 AD which brought an end to the Byzantine Empire.