On September 4, 476, Romulus Augustulus, the last Western Roman Emperor, abdicated after forces led by Odoacer invaded Rome. It is considered the traditional end of the Western Roman Empire. From the article:
"The Dissolution of the Empire
Even so, the two halves of the empire continued to prosper equally until the reign of the Emperor Theodosius I (379-395 CE) when internal and external forces exerted themselves to break the two halves apart. These forces included, but were not limited to:
•Political instability
•The self-interest of the two halves
•Invasion of barbarian tribes
•Government corruption
•Mercenary armies
•Over-reliance on slave labor
•Massive unemployment and inflation
•The rise of Christianity
As noted, Eastern and Western Rome pursued their own interests instead of working in concert toward shared goals. This lack of cohesion fostered political instability which was made worse by government corruption, especially among provincial authorities who abused their positions for personal gain. The Goth and Hun mercenaries in the Roman army owed no allegiance to Rome, they were just fighting for pay and, further, were not treated as well as they felt they deserved. The over-reliance on slave labor took jobs away from the lower classes who then relied on public assistance and the earlier debasement of the currency under Septimus Severus had become a policy of later emperors, resulting in inflation.
Theodosius I's zeal in spreading Christianity and crushing pagan influences has also been noted as a contributing cause to the fall of Rome. The earlier pagan belief system of the Romans had been a state religion; the faith informed the state, and the state supported the faith. The Roman gods were concerned with Rome and its success; the new Christian god had no vested interest in Rome per se and was the god of everyone. The nature of Christianity, according to some scholars, served to weaken the traditional cohesion Roman paganism provided to the empire. That point has been debated for centuries but Theodosius I's persecution of the pagans is a far more certain factor since, as emperor of both the east and the west, he had the power to unite the Roman Empire but, instead, divided it further through religious intolerance.
Theodosius I came to power following a number of serious setbacks for Rome. The Gothic War of 376-382 CE severely weakened the Western Empire even though the battles were routinely fought by forces from the Eastern Empire. At the Battle of Adrianople in 378 CE, the Eastern Emperor Valens (r. 364-378 CE) was defeated by Fritigern (d. c. 380 CE) of the Goths and many historians agree that this marks the beginning of the end of the Roman Empire. The emperor Gratian (of the Western Empire, r. 367-383 CE) had elevated Theodosius to co-emperor status and, when he died, Theodosius I became emperor of both halves of the empire. Theodosius I's treatment of the mercenary Goths – especially at the Battle of the Frigidus in 394 CE – provoked the Goth king Alaric I (r. 395-410 CE) to sack Rome in 410 CE.
This is not at all to claim that Theodosius I's reign led to the end of the Western Roman Empire. There is no single cause for the decline and fall of Rome. A steady deterioration in power and prestige had been ongoing prior to the Roman defeat at Adrianople and all these challenges and pressures culminated in the deposition of the emperor Romulus Augustulus (r. 475-476 CE) by the Germanic king Odoacer on 4 September 476 CE, prior to Adrianople. The Western Roman Empire, essentially, fell with the rise of Odoacer who ushered in a new era which would see the Kingdom of Italy replace the power of Rome in the west.
The Kingdom of Italy
While c. 476 CE is the traditionally accepted date for the end of the Western Roman Empire, that entity did continue on under the rule of Odoacer (r. 476-493 CE) who, officially anyway, was simply ruling in place of the deposed emperor Julius Nepos (who had been deposed by the general Orestes who had placed his son, Romulus Augustulus, on the throne). Therefore, there are historians and scholars who date the end of the Roman Empire to the assassination of Julius Nepos in 480 CE. After Nepos' death, Odoacer annexed the region of Dalmatia to his own lands which concerned the emperor of the eastern part of the empire, Zeno (r. 474-475, 476-491 CE), by whose authority Odoacer had been allowed to rule. In Zeno's view, Odoacer was acting with too much independent authority and was beginning to pose a significant threat.
His suspicions were confirmed when Odoacer was found to be backing Zeno's rival, the general Illus, in a revolt. Zeno employed the Gothic leader Theodoric (later known as Theodoric the Great, r. 493-526 CE) to defeat Illus but then Theodoric turned his formidable army on Zeno and Constantinople. Scholar Guy Halsall explains:
The Goths threatened Constantinople and ravaged the Balkans but could not take the capital, whilst Zeno, secure behind the city's famous triple line of walls, was unlikely to drive the latter completely from his territories. A solution was required, agreeable to both parties, and found: for Theodoric's Ostrogoths to move to Italy and dispose of the 'tyrant' Odoacer. (287)
Theodoric invaded Italy at the head of his army in 488 CE and battled the forces of Odoacer across the region for the next four years. A compromise was finally brokered by John, Bishop of Ravenna, by which Odoacer and Theodoric would jointly rule but, at the feast to celebrate the end of hostilities in 493 CE, Theodoric assassinated Odoacer and claimed the kingship for himself.
Theodoric's reign brought order and prosperity to the region in the form of laws, building projects, and increased food production but, after his death in 526 CE, his successors could not hold the kingdom together as well. The Eastern emperor Justinian I (r. 527-565 CE) asserted his control over the Kingdom of Italy and met the greatest resistance from the Ostrogoth king Totila (r. 541-552 CE) who claimed the right to the same autonomy Theodoric had won from Rome. Justinian I sent the famous general Belisarius (l. 505-565 CE) to Italy but even he could not outsmart or outfight Totila. The general Narses (l. 480-573 CE) finally defeated Totila at the Battle of Taginae in 552 CE, returning Italy to Roman authorities until the invasion of the Lombards in 568 CE."