[File:Roman Republic Empire map.gif|thumb|Animated map of the Roman Republic and Empire between 510 BCE and 530 CE |alt=Animated map of the Roman Republic and Empire]]

The causes and mechanisms of the fall of the Western Roman Empire are a historical theme that was introduced by historian Edward Gibbon in his 1776 book The Decline and Fall of the Roman Empire. He started an ongoing historiographical discussion about what caused the Fall of the Western Roman Empire, and the reduced power of the remaining Eastern Empire, in the 4th–5th centuries. Gibbon was not the first to speculate on why the Empire collapsed, but he was the first to give a well-researched and well-referenced account. Many theories of causality have been explored. In 1984, Alexander Demandt enumerated 210 different theories on why Rome fell, and new theories emerged thereafter. Gibbon himself explored ideas of internal decline (the disintegration of political, economic, military, and other social institutions, civil wars) and of attacks from outside the Empire. "From the eighteenth century onward," historian Glen Bowersock wrote, "we have been obsessed with the fall: it has been valued as an archetype for every perceived decline, and, hence, as a symbol for our own fears."