Hannibal's victory at the Battle of Cannae, 216 BCE.
Decisive Battles
Season 1 Episode 8: The Gothic Invasion of Rome
Battle of Adrianople and the Sack of Rome AD 378 to 410: Corruption drove the hungry horde of Visigoths to rebel against Rome and pride drove the Emperor Valens, heading a fractured Roman Empire, to take them on without support.