What caused the final collapse of the western roman empire?

The major cause of the fall of the western part of the Roman Empire was the invasions by the Germanic peoples. The Romans lacked unity and were unable to fend off these invasions. These peoples, and two more Germanic peoples, who had been allowed to settle in parts of the empire, took over lands in this part of the empire. Eventually all of its lands were seized, except for Italy. At the end of this process, even Italy was taken over by the Ostrogoths, who were sent there by the eastern Roman Emperor to depose a usurper.