For the century since Diocletian, generally capable and energetic emperors had ruled the empire. However, the death of Theodosius the Great in 395 C.E. marked a turning point in Roman history as the Western half of the empire steadily slipped into oblivion. There were several reasons why the West fell and the East survived. First of all, the East, with its older civilizations and more established trade routes, was considerably richer than the West, so it could buy off the barbarians until it found the strength to fight them.
The second factor was the barbarization of the Roman army. Depopulation and centuries of peace made it hard to get enough qualified recruits for the army. As a result, the Romans had turned more and more to enlisting Germanic tribesmen in their ranks. In the East, there were still areas where good native recruits could be found to balance out the number of barbarians. The West, having few good native recruits, relied more heavily on Germanic recruits. By 400 C.E., they made up an estimated half of the Roman army in the West and ruined its effectiveness by refusing to submit to Roman discipline. Not only that, but the high military commands were also often held by men from these tribes who spent much of their time intriguing for political power rather than defending the empire.
A third factor was that the West had two large frontiers, the Rhine and Danube, to guard against the barbarians, while the East had only the Danube. Granted, the Eastern Empire also had to deal with Persia, but it was often preoccupied with threats on its own borders, in particular from the Huns. Finally, the East had fairly capable emperors after 450 C.E., while the West never had a good emperor after Theodosius I's death in 395.
Popular imagination tends to see the final collapse of the empire in the West as a cataclysmic wave of Germanic tribes overrunning the Roman world. In fact, it was more a case of barbarians infiltrating a civilized society and destroying it from within. The century between the military disaster at Adrianople and the final collapse of the empire in the West did not see a single major victory of barbarians over a Roman army. Instead, in some cases, the Romans freely let in individuals or even whole tribes, which was the case with the Visigoths in 376. In other cases, tribes just walked in when legions were pulled from a frontier to revolt or meet an invasion or revolt elsewhere. That is how such tribes as the Angles, Saxons, Jutes, Visigoths, and Alans got into the empire.
Once inside the empire, the tribes would loot and pillage, but they were also anxious to gain legal status and Roman titles. In the case of a few exceptions, such as the Saxons who had little previous contact with Roman civilization, the invading tribes wanted to become a legal part of the Roman Empire, not destroy it. Long after the Empire in the West was gone, the legal fiction of its existence persisted, both at the Eastern Empire's court in Constantinople, and among the peoples who settled in the West. In fact, the idea of the Roman Empire was so strong among these people that in 800 C.E., three centuries after its fall, the imperial title was revived in the West. The Holy Roman Empire, as this revival of Roman grandeur was called, lived on at least as an idea for 1000 years. Finally in 1806, Napoleon declared the Holy Roman Empire dead, largely to make room for his own imperial ambitions with Roman style titles and military standards. The idea of Rome did not die easily.
This triggered a pattern of events much like the cycle of anarchy in the third century C.E., only this time, no Aurelian or Diocletian emerged to save the Empire. Once a tribe was in the empire, it would loot and pillage, wrecking the empire's economy and lowering its tax base. The increased military burden and decreased means to meet it would weaken the empire's ability to provide an adequate defense, causing more tribes to break in and repeat the pattern. Thus the Visigoths, Vandals, Saxons, Huns, and Franks in turn would benefit from this cycle and also perpetuate it, allowing the next people to come in, and so on.
The Visigoths who started this cycle managed to sack Rome in 410. Pulling troops from the Rhine frontier to meet this threat allowed the Vandals and other tribes to invade Gaul, Spain, and eventually North Africa. The loss of North Africa meant the peace and unity of the Mediterranean were disrupted, further stretching Rome's dwindling defenses and resources. In 455, the Vandals sailed to Italy and sacked Rome in much worse fashion than the Visigoths had. Meanwhile, all this turmoil plus an attempt by a rebellious general, Constantine, to seize the throne had stripped Britain of its legions, and the Angles, Saxons, and Jutes started crossing the Channel. At this point, Britain virtually dropped from the sight of recorded history.
By 450, the Western Empire's material resources were so depleted that there was little or nothing that could save it. When Attila the Hun demanded a huge tribute from the Western Empire, Rome did manage one final military victory in alliance with the Visigoths and other tribes against the much more dangerous Huns. Attila's death soon afterward led to the break-up of his empire, which unleashed his subject tribes against Rome. While Germanic generals in Italy intrigued against one another, setting up puppet emperors in rapid succession, the decrepit remains of the Western Empire came crashing down, and various tribes came pouring in to carve out new kingdoms on its ruins. The last and, as it would turn out, most important tribe, the Franks, now started to make its move to carve out its own kingdom in northern Gaul. As it turned out, the Franks would be the tribe to contribute the most to the transition from the ancient world to Western Civilization.
The last legally recognized emperor of the West, Julius Nepos, died in exile in 480. Although the eastern emperors in Constantinople claimed that they now ruled over the whole empire, for all intents and purposes the Roman Empire in the West was gone. The Dark Ages would descend upon the West, while the Eastern Roman, or Byzantine, Empire managed to survive, revive, and attain new heights of its own in the centuries ahead. The heritage of antiquity would live on, but a new era in history was dawning: the Middle Ages.