Military leaders usurped power, rulers died of unnatural causes, revolts, plagues, fires, Christian persecutions.
Diocletian splits the Roman Empire in 2, and adds junior emperors, so there are 4 caesars. When Diocletian and Maximian abdicate, there is civil war.
In 312, Constantine defeats his co-emperor at the Milvian Bridge, and becomes sole ruler in the West. Later Constantine defeats the Eastern ruler and becomes sole ruler of the Roman Empire. Constantine establishes Christianity and creates a capital for the Roman Empire in the East, in Constantinople.
Julian the Apostate attempts to reverse the religious trend to Christianity. He fails and dies in the East fighting the Parthians.
Eastern Roman Emperor Valens is defeated by the Visigoths.
Theodosius reunites the Empire, but it doesn't last beyond his reign. At his death, the empire is divided by his sons, Arcadius, in the East, and Honorius, in the West.
Visigoths make incursions into Italy and in the end, under Alaric, sack Rome. This is one date given for the Fall of Rome.
Vandals, under Gaiseric, attack northern Africa, cutting off Roman grain supply.
Huns threaten Rome, are paid off, and then attack, making Rome and the army weak.
Vandals plunder Rome but, by agreement, injure few people or buildings.
Last western emperor, Romulus Augustulus, is deposed by the barbarian general Odoacer who then rules Italy.