This date is generally considered to mark the end of antiquity and the beginning of the Middle Ages. A.D. 410 was the year that Rome fell to the Visigoths.
Back in 382, the nominally Roman emperor Theodosius I (who actually ruled from Constantinople) signed a treaty with the Visigoths that made the Visigoths part of the Roman empire. While the Romans considered the Visigoths now part of the empire, the Visigoths thought differently. They despised Rome and thought of themselves as independent of it.
It was the general Flavius Stilicho's job to keep the Visigoths under Alaric I out of Italy. Alaric, however, persuaded Stilicho that if he would help Alaric, Alaric would help him capture Illyricum (basically Greece and Macedonia) for the Western empire. While they were preparing for the invasion, Alaric crossed the Rhine and invaded Gaul. So Stilicho called the invasion of Illyricum off and stuck Alaric with the bill. At that point, both Stilicho and Alaric thought that the other had double-crossed them.
In 408, Honorius, son of Theodosius I and the Western Roman Emperor, got into the act. Honorius and Stilicho had a fight about whether or not Stilicho would allow Honorius to go to Constantinople. Things escalated until Stilicho was deposed, took refuge in a church, but was hewn down anyway. The reprisals against Stilicho's followers and slaves who had been brought to Italy by him resulted in many of them fleeing to Alaric, swelling his army. By the end of 408 the Visigoths invaded Italy and besieged Rome.
The Senate tried to get Honorius to settle with Alaric, but Honorius kept reneging on his agreements and insulting Alaric.
Finally, on 24 August 210, the Roman slaves opened the gates to the Visigoths who plundered Rome for three days.
The Visigoths thus plundered the Roman empire from the inside. Rome probably could have been spared were it not for the treachery and double dealing of the ironically named Honorius.
