The Battle of Agrigentum was the first pitched battle of the First Punic War and the first large-scale military confrontation between the Roman Republic and Carthage.
In 265 BC, the Mamertine mercenaries in control of Messana sent requests for aid from both the Roman Republic and Carthage as the powerful Greek city-state of Syracuse prepared to put an end to their reign of terror. Carthage was the first to respond, sending a small force to garrison Messana's citadel. The Roman Senate grew concerned that Carthage would utilize this intervention as a means of conquering the whole of Sicily from the Greek states, upsetting the balance of power which had amid centuries of Greco-Punic warfare. The Roman Senate voted to send an expedition, led by Consul Appius Claudius Caudex, to Sicily to deter or defeat any opposition in Sicily. The Mamertines, who had grown uncomfortable under Carthaginian "protection", helped the Romans to oust the Carthaginians from Messana in 264 BC. Neither Rome nor Carthage had declared war, and Carthage made conciliatory gestures toward Rome before increasing its troop presence on Sicily in 262 BC; that same year, the Roman Senate planned to dispatch two legions to the island. The Carthaginians hired Ligurian, Celtic, and Spanish mercenaries and dispatched them to Sicily, where Agrigentum became their main base; the Carthaginians also built up their forces on Sardinia and planned to use the island as a base for attacks on Italia. The Roman consuls Lucius Postumius Megellus and Quintus Mamilius Vitulus led 40,000 Roman troops to Sicily and decided to besiege Agrigentum. The Carthaginians evacuated the people from the surrounding countryside to Agrigentum, swelling its population to 50,000, while the Romans harvested the crops that the Carthaginians had left behind.
The Carthaginians sortied while the Romans harvested crops in the fields, inflicting heavy losses on the outnumbered and unarmed foragers. However, the Romans were able to fight off a Carthaginian attack on their camp, and Hannibal Gisco decided against attempting a second attack. The Romans proceeded to dig a series of ditches and small forts around the city, and, by November 262 BC, five months of siege had taken their toll on Agrigentum's supplies. Hannibal sent for reinforcements from Africa, so the Carthaginian general Hanno brought 50,000 infantry, 6,000 Numidian cavalry, and 60 elephants to come to Agrigentum's relief. Hanno cut off the Roman supply lines and lines of communication, and the Numidian cavalry also lured the Roman cavalry into attacking the main Carthaginian column, resulting in heavy losses. Ultimately, it was the turn of the Carthaginians to besiege the Romans. The Roman consuls decided to seek a pitched battle to resolve the stalemate after two more months of skirmishing and siege, while Hannibal Gisco, commanding the beleaguered defenders of Agrigentum, persuaded a reluctant Hanno of the need to bring the Romans to battle and relieve the siege. In the ensuing battle, the Romans discovered the Carthaginian generals' plans for a coordinated attack and thus decided to attack Hanno's army from the side and rear, defeating Hanno's army after a hard-fought and costly battle. The Roman cavalry also attacked the Carthaginian camp, capturing several elephants. Hannibal Gisco and the garrison of Agrigentum succeeded in breaking through the Roman line and escaping to safety, filling the Roman trenches with straw in order to enable a quick evacuation. The Romans proceeded to enter Agrigentum unopposed, and they plundered the city and sold 25,000 of its inhabitants into slavery. However, the Romans' cruel punishment of the Agrigentines hardened other Sicilian cities' opposition to cooperation with the Romans, and the Roman consuls were not awarded a triumph due to the escape of the Carthaginians and their heavy losses.