Seeds of the ultimate collapse of the west were planted during the third century crisis and persisted throughout the relative stability brought by Diocletian/Constantine. The borders were secure again, but within those borders rome was dying. The population losses from half a century of plague and constant warfare forced Emperors to settle Germanic tribes within Roman borders, and as the Legions were decimated by extremely bloody civil war battles between the Tetrarchy, then Constantine and his rivals, then Constantine's sons, then Thedosius, etc. commanders were forced to rely more and more on foederati who were never fully integrated into Roman society and mercenaries. After Adrianople and the invasions of 406 Rome had to give territory, money, and military positions to Tribal leaders just to secure their loyalty, and threats outside of the empire had to be increasingly paid off with money the Emperors didn't have. Inflated currencies and overtaxation forced free citizens to become tenant farmers under rich landlords, and diocletian's reforms mandating hereditary occupations kept the population immobile and at the mercy of local magistrates. Collecting taxes from the rich landholders was difficult, and the split between east and west cut the western half of the empire off from the massive revenues generated by wealthy provinces like Syria and Egypt. Without money you can't pay for large armies, without large armies you can't protect the frontiers. When the people living in those frontiers realize they're paying taxes and not being protected they turn to the Germanic tribes that moved in and pay taxes to them instead.