The Western Roman Empire ceased to exist in 476 when the last Western Roman Emperor, Romulus Augustulus was deposited by Germanic chieftain Odoacer. However, the event itself was not the cause for the empire's collapse but a result of a process that started in the second half of the 2nd century. In the 3rd century, the Roman Empire nearly collapsed under pressure of barbaric invasions, civil war and economic crisis known as the Crisis of the 3rd Century.
The Roman territorial expansion came to an end, while its frontiers were threatened by Sassanid Empire on the east and Germanic peoples on the north and northeast. The external threat increased the costs for defense but at the same time, the income from the Roman provinces started to decline. Due to lack of financial sources the costs for administration and bureaucracy continued to grow and further deepened the economic crisis that started to endanger the integrity of the Roman Empire.
Economic crisis was also a result of lack of slave labor which in turn was a result of changed Roman policy. The Roman economy was greatly depended on slave labor but the majority of slaves was acquired through territorial expansion. Lack of slave labor force directed in major social changes in the countryside where dramatically increased the number of coloni, tenant farmers working on large estates and paying a rent to the owners in exchange. The Roman Emperors were unable to put an end to the crisis which also saw the rise of Christianity.
The Roman Empire managed to survive the Crisis of the 3rd Century due to reforms of Diocletian and Constantine the Great. However, after the permanent division of the empire into western and eastern halves in 395 the Western Roman Empire became unable to repulse the invasions of the barbarian tribes who started penetrating deeper into the Roman territory. Occasionally, the Roman Emperors were forced to allow the barbarians to settle on the Roman territory in exchange for providing military assistance (foederatus).
A large part of the Western Roman Empire was held by barbarians as foederati by the mid-5th century and the barbarian military leaders ever increased their influence in the empire. The Western Roman Emperors held authority only formally after the mid-5th century, while the real power was in the hands of barbarian military leaders. Odoacer's seizure of the throne in 476 was there a “natural” output of a process which can not be attributed to barbarian invasions alone