Not one to let the problem of half an army get in the way of his conquests, Hannibal set off into Italy. In the Battle of the Trebia, the outnumbered Carthaginians convinced the Romans to seek battle before they'd even had their breakfast. The Carthaginian forces were well-rested and well-fed, unlike the Romans, who proved little match. Eager to get revenge, Gaius Flaminius marched north with another army. What followed was arguably one of the greatest ambushes in military history - the Battle of Lake Trasimene. Hannibal lured the Romans onto a road, with forested hills to their left and Lake Trasimene to their right. Hidden in these hills, masked by fog, were the Carthaginian soldiers, who quickly descended on Flaminius' legions, forcing them into the lake. The destruction of this second army led the Romans to adopt a policy of attrition, avoiding pitched battles and hoping to wear the enemy down through other means. This left Hannibal free to walk his way around Italy, sowing discord and discontent. Hannibal soon tired of this, and was desperate to face Rome in the field once again. To make this happen, he captured the Roman supply depot at Cannae. The ensuing Battle of Cannae resulted in one of the largest losses of life of any battle in the ancient world. According to Livy:
There was no longer any Roman camp, any general, any single soldier in existence.- Livy 22.54
This was not entirely true, but to the Romans it must surely have felt that way. How then did the Romans claw their way back from the apparent jaws of defeat, push Hannibal back to Africa, and give him a right walloping? Rather conveniently for the purposes of this blog, I think that the answer lies in food.
Food vs Force
In 209 B.C. Rome fought hard to capture the city of New Carthage in Spain, but "amidst such an enormous supply of military and naval stores, the actual city itself was regarded as the least important capture of all." (2) This quote is great because it shows that Carthage had wealth enough to fund, feed, and support countless armies. The problem was that this wealth was in Spain, and not in Italy with Hannibal. Furthermore, to make good time when crossing the Alps, Hannibal had his soldiers leave whatever food, weapons, and wives they did not need behind in France. (3) The several tribes of angry Gauls he fought and left behind before made sure that these supplies could not reach Italy. So, here we have Hannibal stuck in Italy, separated from the very supplies he needs to wage war - what gives?
Hannibal left his brother Hasdrubal and his general Hanno the ships and soldiers necessary to control the Mediterranean coastline, meaning that he could be resupplied by sea. (4) This was only part of the plan however. Hannibal's overarching strategy was not to crush Rome militarily (which he did with great gusto), but to convince the rest of Italy to turn against their Roman overlords. The Roman world was not, at this stage, as unified as you might expect, and it was only recently that vast swathes of Italy had come under Roman control. Hannibal hoped to undermine this control by showing the Italians that the Romans could be defeated. He hoped that the Italians would join the cause and rush to his side, bringing with them the soldiers and sandwiches (Note: This is ENTIRELY anachronistic, but it illustrates a point) needed to subdue the enemy. This didn't happen.
As much as the Italians disliked Rome, at least the Romans shared the same gods, customs, and sense of dress as them. The Carthaginians on the other hand had strange gods and liked to sacrifice children (or so says late Roman propaganda, so take this with a pinch of salt/ dash of garum). (5) Hannibal's anticipated support never fully materialised, and his hopes of resupply from Spain by sea were dashed when Rome destroyed the Carthaginian navy. Stranded in Italy with thousands of soldiers to feed and pay, Hannibal had to start stealing food from the very same Italians he hoped would join his cause. Things did not look good for Mr. Hannibal Barca.
Things didn't look particularly good for the Romans either. Italy's farmers "had been carried off by the war, there was hardly any slave labour, the cattle had been driven off as plunder, and the farms and houses had been either stripped or burnt." (6) Luckily, thanks to the First Punic War, Rome had found a friend in Hiero of Syracuse, who saw to it that Rome had the grain supplies necessary to get them through the war. (7) Whilst it wasn't particularly tasty or exciting, Rome had food, and Carthage didn't - they could simply sit back and wait for Hannibal's army to starve.
Footnotes1) Livy, 21.38
2) Livy, 26.47
3) Livy, 21.60
4) Polybius, 3.95 and Livy, 21.23
5) Plutarch, On Superstition, 13
6) Livy, 28.11
7) Livy, 23.21