The fall of the Western Roman Empire was a gradual process, but historians generally agree it reached its culmination in 476 AD when Romulus Augustulus--the last emperor of the Western Roman Empire--was deposed by Flavius Odoacer. The main reasons the Western Roman Empire declined were internal unrest and external attacks.
Internal problems weakened Rome. Many Roman emperors were poor leaders. The growth of Christianity also brought a challenge to the divinity and authority of the Roman emperors and their governments, and several emperors experienced challenges to their power from rivals who wanted to usurp the throne.
Rome also experienced external attacks. In 376, Goths began to migrate West as they escaped from the Huns. By 410, the Visigoth king Alaric had sacked Rome. Continued attacks and internal struggles continued to weaken Rome until Romulus Augustulus' deposition in 476.