On August 2, 216 BC, during the Second Punic War at the Battle of Cannae, the Carthaginian army led by Hannibal defeated the Roman. An excerpt from the article:
"The Battle of Cannae (2 August 216 BCE) was the decisive victory of the Carthaginian army over Roman forces at Cannae, southeast Italy, during the Second Punic War (218-202 BCE). The Carthaginian general Hannibal Barca (l. 247-183 BCE), who was already known for his unorthodox tactics against Rome, counted on the Romans relying on the traditional tactics and formations which had worked so well for them in the past and used their very strengths to defeat them.
In 216 BCE, Roman military tactics were still in their infancy. Although Rome had won many impressive victories during the First Punic War (264-241 BCE) they continued to rely on their old tactic of placing a numerically superior force in the field to overwhelm the enemy. The typical Roman formation was to position light infantry toward the front masking the heavy infantry and then coordinating light and heavy cavalry on the back wings.
This formation had worked well for Rome's earlier adversary, the Greek King Pyrrhus I (r. 297-272 BCE) who, although victorious at the Battle of Asculum in 279 BCE, lost so many men that his army could not continue on to take the city (giving rise to the phrase 'Pyrrhic victory'). Pyrrhus' basic strategy was effective, however, and was adopted in Roman warfare: he would place a large force in the field, in tight formation, and rely on the superior numbers and the charge to break the opposing ranks. This tactic had worked for the Romans during the First Punic War but, at the Battle of Cannae, the Romans would learn an important lesson in military strategy from a general who fought like no other had before him and would leave a lasting legacy for Rome and the wider world."