The first sack of Rome was in 410 by Alaric of the Goths. However, Alaric had not invaded the Roman empire with the intention of sacking Rome. He was a Christian who entered the Roman empire looking for a place to settle his people inside the Roman empire. After Alaric crossed into the Roman empire it appears that he made some deals with the Roman general Stilicho who used the Goth army for his own advantages against outside forces. When Stilicho was deposed due to Roman politics Alaric found that he had no friends among the emperor or elite Roman Senators. After negotiations for compensation for the work he had down failed Alaric then took his compensation in the sacking of Rome.
(Byzantium: The Early Centuries – John Julius Norwich)