Fall of the Western Empire
After 180 AD and the end of Pax Romana the Roman Empire entered a period of decline. The empire tore itself apart by internal civil wars while at the same time trying to protect her borders from barbarian invasions. In 410 AD the eternal city herself was sacked by a gothic army under the warlord Alaric. For three days the goths looted, burned, plundered and pillaged. It was the first time in over 1000 years that Rome was not in the control of romans but it was not to be the last.
Even though the Goths were demonized for the sacking, it was a gentle one by the standards of the day and the Goths were in fact a Roman Army (made up of Goths) who had not been paid for the services and lands promised for their tribes had not been delivered.
Even as Rome was under attack from external forces it was also crumbling from an internal financial crisis. Losing lands within the Empire to the Goths and Franks lightened the imperial coffers. Rome at her height and most powerful was expanding adding new lands and people to the empire, now the opposite was happening. The fate of the West was sealed when in the 3rd century the Emperor Diocletian divided the empire into the East and West, One ruled from Milan and the other ruled from Constantinople. This was to make the empire more governable but the two halves gradually started to drift apart one Latin (West) and one Greek (the East). So as the West declined the East grew and flourished.
The decline of Rome dovetailed with the spread of Christianity and many believe that Rome's fall was quickened by the rise of the new religion. It (Christianity) became the state religion in 380 AD and eroded the Traditional Roman values where the Roman Emperor was looked upon as divine.
The glory of Rome was replaced by the glory of Christ and church leaders assumed an increasing role in political affairs complicating things. The Emperor never had to answer to anyone (except the mob) but now he had to answer to the church and her leaders.
But the greatest reason why Rome fell was the decline in her legions. For most of her history Rome's military was the envy of the ancient world but no longer. Unable to recruit Romans (some Romans cut their thumbs off to avoid conscription) for the army, the emperors started recruiting foreign mercenaries to prop up the numbers. The ranks swelled so much with barbarians that the Romans began using the word "barbarus" in place of soldier. While they proved to be fierce warriors they held little loyalty to Rome or the Emperor and their power hungry leaders often turned against their Roman employers.
The End in the West
One such mercenary leader was the Germanic Goth Odoacer who deposed the last Roman emperor in the West "Romulus" in 476 AD and founded a romanized Gothic kingdom in Italy. The Vandals had taken North Africa (main source of the Empire's food) in 410 AD and Sacked Rome in 453 AD leaving the great city only a shadow of its former self.
The Roman empire in the West had fallen but the Greek Eastern empire would endure for a further 1000 years seeing many ups and downs over the following centuries. The Byzantine (as the Eastern empire was now known) Emperor Justinian with help from the great general Belisarius briefly conquered North Africa and Italy in the 530's but a reunification of the old empire was not to be and was just a dream. The Byzantine Empire would finally fall to the Ottoman Turks in 1453 AD