Second Punic War in 217. Moving to southern Italy in 216, Hannibal at Cannae annihilated the largest army the Romans had ever assembled. After the death or imprisonment of 130,000 Roman troops in two years, 40% of Rome's Italian allies defected to Carthage, giving her control over most of southern Italy. Macedon and Syracuse joined the Carthaginian side after Cannae and the conflict spread to Greece and Sicily. From 215–210 the Carthaginian army and navy launched repeated amphibious assaults to capture Roman Sicily and Sardinia but were ultimately repulsed. Against Hannibal's skill on the battlefield, the Romans adopted the Fabian strategy – the
Early Middle Ages had broken down, and the Roman army comprised mainly barbarians and soldiers recruited for a single campaign. The general decline in discipline also led to the use of smaller shields and lighter weaponry. Not wanting to share the glory, Eastern Emperor Valens ordered an attack on the Therving infantry under Fritigern without waiting for Western Emperor Gratian, who was on the way with reinforcements. While the Romans were fully engaged, the Greuthung cavalry arrived. Only one-third of the Roman army managed to escape. This represented the most shattering defeat that the Romans had suffered since the Battle of Cannae (216