Punic Wars
Roman allies defect, 216–207 BC

Little survives of Polybius's account of Hannibal's army in Italy after Cannae and Livy is the best surviving source for this part of the war. Several of the city states in southern Italy allied with Hannibal or were captured when pro-Carthaginian factions betrayed their defences. These included the large city of Capua and the major port city of Tarentum (modern Taranto). Two of the major Samnite tribes also joined the Carthaginian cause. By 214 BC the bulk of southern Italy had turned against Rome, although there were many exceptions. The majority of Rome's allies in central Italy remained loyal. All except the smallest towns were too well fortified for Hannibal to take by assault and blockade could be a long-drawn-out affair, or, if the target was a port, impossible. Carthage's new allies felt little sense of community with Carthage, or even with each other. The new allies increased the number of places that Hannibal's army was expected to defend from Roman retribution, but provided relatively few fresh troops to assist him in doing so. Such Italian forces as were raised resisted operating away from their home cities and performed poorly when they did.

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 that 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 the Carthaginian defeat there at the battle of Dertosa.

Meanwhile, the Romans took drastic steps to raise new legions: enrolling slaves, criminals and those who did not meet the usual property qualification. By early 215 BC they were fielding at least 12 legions; by 214 BC, 18; and by 213 BC, 22. By 212 BC the full complement of the legions deployed would have been in excess of 100,000 men, plus, as always, a similar number of allied troops. The majority were deployed in southern Italy in field armies of approximately 20,000 men each. This was insufficient to challenge Hannibal's army in open battle, but sufficient to force him to concentrate his forces and to hamper his movements.

For 12 years after Cannae the war surged around southern Italy as cities went over to the Carthaginians or were taken by subterfuge and the Romans recaptured them by siege or by suborning pro-Roman factions. Hannibal repeatedly defeated Roman armies, in 209 BC both consuls were killed in a cavalry skirmish. But wherever his main army was not active the Romans threatened Carthaginian-supporting towns or sought battle with Carthaginian or Carthaginian-allied detachments; frequently with success. By 207 BC Hannibal had been confined to the extreme south of Italy and many of the cities and territories which had joined the Carthaginian cause had returned to their Roman allegiance.
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