Having recovered from their losses at Trebia (218 BCE) and Lake Trasimene (217 BCE), the Romans decided to engage Hannibal at Cannae, with approximately 86,000 Roman and allied troops. They massed their heavy infantry in a deeper formation than usual, while Hannibal used the double envelopment tactic and surrounded his enemy, trapping the majority of the Roman army, who were then slaughtered. The loss of life on the Roman side meant it was one of the most lethal single days of fighting in history. Only about 15,000 Romans, most of whom were from the garrisons of the camps and had not taken part in the battle, escaped death. Following the defeat, Capua and several other Italian city-states defected from the Roman Republic to Carthage.

As news of this defeat reached Rome, the city was gripped in panic. Authorities resorted to extraordinary measures, which included consulting the Sibylline Books, dispatching a delegation led by Quintus Fabius Pictor to consult the Delphic oracle in Greece, and burying four people alive as a sacrifice to their gods.