Vandals

On June 2, 455, King Gaiseric and the Vandals sacked Rome. From the article:

"The Sack of Rome
From their port at Carthage, the Vandals now launched their fleet at will and controlled the Mediterranean Sea, which formerly had been Rome's. Gaiseric's navy plundered whatever ships crossed its path and raided coastlines. Plans and attempts by the Romans to drive Gaiseric and his people from North Africa came to nothing, and so, in 442 CE, the Romans acknowledged the Vandal kingdom as a legitimate political entity and a treaty was signed between Gaiseric and Valentinian III.

In 455 CE, Valentinian assassinated Aetius and was then murdered shortly afterwards by Petronius Maximus. Gaiseric claimed that this nullified the treaty of 442 CE, which had been only valid between himself and Valentinian. He sailed for Italy with his fleet, landed unopposed at Ostia, and marched on Rome. The Romans recognized that their military force was inadequate to meet the Vandals and so put their trust in the diplomatic skills of Pope Leo I (served 440-461 CE) and sent him out to meet Gaiseric and plead for mercy.

Leo told Gaiseric he was free to plunder the city but asked him not to destroy it nor harm the inhabitants - and Gaiseric agreed. This was greatly to Gaiseric's advantage on many points, but mainly because Italy was suffering a famine and, when he landed at Ostia, Gaiseric recognized that his army would be unable to affect a prolonged siege of the city because they would have nothing to eat and Rome's walls were formidable. His assent to Leo's request, then, was more an act of expediency and prudence than mercy.

Anything of value, from personal treasures to ornaments on buildings and statues, was taken by the Vandals, but they did not destroy the city and few people were harmed other than Petronius Maximus, who was killed by a Roman mob when he tried to flee and was caught outside the walls. The Vandals looted the city and then marched back to their ships and sailed home, taking with them a number of high-profile hostages including Valentinian III's widow and her daughters. Collins writes:

The sack of Rome of 455 had the immediate effect of making the Vandal threat to Italy seem far more menacing than [other threats]. Despite the Vandals immediately returning to Africa with their loot, the whole episode brought home in a day that seems not to have been previously appreciated just how vulnerable Italy, and Rome in particular, was to sea-borne raiding. (88)"