Several of the city states in southern Italy allied themselves with Hannibal, or were captured when pro-Carthaginian factions betrayed their defences. Two of the major Samnite tribes also joined the Carthaginian cause. By 214 BC the bulk of southern Italy had turned against Rome.

The greatest gain was the second largest city of Italy, Capua, when Hannibal's army marched into Campania in 216 BC. The inhabitants of Capua held limited Roman citizenship and the aristocracy was linked to the Romans via marriage and friendship, but the possibility of becoming the supreme city of Italy after the evident Roman disasters proved too strong a temptation. The treaty between them and Hannibal can be described as an agreement of friendship, since the Capuans had no obligations. When the port city of Locri defected to Carthage in the summer of 215 BC it was immediately used to reinforce the Carthaginian forces in Italy with soldiers, supplies and war elephants. It was the only time during the war Carthage reinforced Hannibal. A second force, under Hannibal's youngest brother Mago, was meant to land in Italy in 215 BC but was diverted to Iberia after a major Carthaginian defeat there.