Battle of Cannae - Wikipedia

On August 2, 216 BC, during the Second Punic War, the Battle of Cannae took place. TheCarthaginian army led by Hannibal defeated
a numerically superior Roman army under the command of consuls Lucius Aemilius Paullus and Gaius Terentius Varro. From the article:

"Shortly after the start of the Second Punic War, Hannibal crossed into Italy by traversing the Pyrenees and the Alps during the summer and early autumn. He quickly won major victories over the Romans at Trebia and at Lake Trasimene. After these losses, the Romans appointed Quintus Fabius Maximus Verrucosus as dictator to deal with the threat.[1] Fabius used attrition warfare against Hannibal, cutting off his supply lines and avoiding pitched battles. These tactics proved unpopular with the Romans who, as they recovered from the shock of Hannibal's victories, began to question the wisdom of the Fabian strategy, which had given the Carthaginian army a chance to regroup.[2] The majority of Romans were eager to see a quick conclusion to the war. It was feared that, if Hannibal continued plundering Italy unopposed, Rome's allies might defect to the Carthaginian side for self-preservation.[3]

Therefore, when Fabius came to the end of his term, the Senate did not renew his dictatorial powers and command was given to consuls Gnaeus Servilius Geminus and Marcus Atilius Regulus. In 216 BC, when elections resumed, Gaius Terentius Varro and Lucius Aemilius Paullus were elected as consuls, placed in command of a newly raised army of unprecedented size and directed to engage Hannibal. Polybius wrote:

The Senate determined to bring eight legions into the field, which had never been done at Rome before, each legion consisting of five thousand men besides allies. ...Most of their wars are decided by one consul and two legions, with their quota of allies; and they rarely employ all four at one time and on one service. But on this occasion, so great was the alarm and terror of what would happen, they resolved to bring not only four but eight legions into the field.

— Polybius, The Histories of Polybius"