Some of the reasons that historians give for the fall of the western Roman Empire include barbarian migration and invasion, economic problems, the growing power of the Eastern Empire, overexpansion and military overspending, political corruption and instability, the rise of Christianity, and the weakening of the Roman legions.
Many historians put the fall of Rome as happening at some time during the fifth or sixth century. Historians disagree as to what was the ultimate cause of Rome's demise, and so they each ascribe different dates that correspond to their own particular theories. Some see the end as the result of the Visigoth king Alaric's sack of Rome in 410, whereas others date it to Odoacer's deposition of the last Roman emperor in 476. Still others tie the Empire's ultimate collapse to Eastern Emperor Justinian's failed attempt to regain control of the West.
Those who ascribe the fall to the weakening of the Roman legions point to the empire's increasing use of barbarian mercenaries instead of Roman soldiers. These mercenaries had little connection to the empire besides fighting for it, and so were sometimes inclined to turn on Rome. Many scholars also attribute the empire's fall to the legions of foreign barbarians who invaded Roman territory, such as the Huns.