The Western Roman Empire declined very slowly over hundreds of years instead of suddenly falling apart. As early as the fifth century A.D., Rome had divided into two parts — the Eastern (or Byzantine) and the Western. The Western half of Rome experienced increasing pressure from within and without during those last hundred years of its existence. What had begun as an incredibly powerful entity ultimately succumbed to both internal structural weakness and the constant influx of outside forces.

A significant contributing factor to the decline was economic stress. Heavy taxation of peasants and merchants supported the armies and bureaucracies of the Roman Empire, which contributed to severe inflation. Because of devaluation of the currency by emperors, trade networks deteriorated; cities diminished; and large agricultural holdings became virtually independent "feudal" type entities. Small-scale landowners became increasingly indebted, losing possession of their lands. Additionally, the bureaucracy expanded exponentially, becoming increasingly corrupt as officials skimmed what they could.

Military issues did not help either. The once disciplined legions of Rome employed ever-increasing numbers of foreign mercenaries, including the barbarian warriors that were recruited from the surrounding areas. Loyalty to Rome varied among these foreign fighters based upon their personal interests, and some fought against Rome whenever doing so benefited them. The military forces also continued to be attacked by external enemies. The Visigoths crossed the Danube River in the late 300's AD and invaded Europe to escape persecution from the Huns from Asia. When Visigoth leader Alaric led a group of Visigoths to sack Rome in 410 AD he shocked the entire civilized world although Rome had already become a shadow of its former self. Other invaders soon followed: Vandals conquered Gallia and Hispania and then moved into North Africa cutting off vital wheat shipments. Attila and his Huns devastated the Balkan Peninsula and northern Italy. Each successive wave of invasions eroded Rome's territory and resources further.

As far as political instability is concerned, there existed widespread chaos at the pinnacle of power. Successive Roman leaders were assassinated or toppled by their generals. The partitioning of the empire in 395 AD resulted in the west being economically impoverished and militarily vulnerable while the east, based around Constantinople, remained prosperous and more stable. The Roman rulers of the west were unable to effectively collect taxes nor repair roads and fortifications.

By the mid-400's AD, the western portion of the empire was nearly completely fragmented from imperial authority. Britain, Gaul, Spain, and Africa were all largely lost or governed by native chiefs. In 476 AD Odoacer, a Germanic warlord, deposed Romulus Augustus, a teenager who had been proclaimed Emperor by his father. While historians commonly date the collapse to that event, ordinary Romans did not experience a sudden loss of civilization. Rather, various parts of the region maintained elements of Roman law and Latin culture for varying periods while other sections developed new hybridized forms of culture.

Historians have debated the relative importance of several factors for decades. Is the fall due to moral corruption, climatic changes, or toxic lead levels? Or is it simply because all great civilizations tend to fail when they cannot adapt? In reality, it was likely a combination of these variables. The eastern portion of the empire would continue to survive for an additional millennium as Byzantium. However, in the west, the fall allowed for the rise of numerous medieval kingdoms whose eventual development will shape modern European society. This illustrates how even great civilizations may be destroyed if they are unable to evolve.