Of the two Consular armies one was at Ariminum (Rimini), nominally watching an enemy who was now busy elsewhere, the other was at Arretium (Arezzo). It was the latter that Hannibal designed to engage, his plans being laid in such a way as to show that his political sagacity was not less remarkable than his military genius. The Roman general was C. Flaminius, a vehement advocate of plebeian rights. He had denounced the incapacity of the Senate and of the patrician generals. He had gained some distinction as a soldier, though, as a matter of fact, his victories had been won by the valour of his troops, which had triumphed in spite of their general's blunders. During his canvass for the Consulship he had loudly proclaimed that, put at the head of the army, he would speedily make an end of the invader. Now the time was come for him to make good his boast. If he had been himself disposed to hang back, though there is no reason for supposing that he doubted of the result, he could not disappoint his friends and followers. The camp was half filled, we are told, with adventurers who had thronged to get a share in the Carthaginian plunder.

Hannibal marched slowly past the Romans, ravaging the country as he went, and Flaminius, infuriated by the sight, immediately broke up his camp and pursued him. The omens were, it was said, of the gloomiest kind. When the Consul mounted his horse the animal stumbled and threw him; when the standard was to be removed all the efforts of the officer whose business it was to take charge of it was unable to stir it from the ground. Flaminius was wholly unmoved by these occurrences, and followed Hannibal in hot haste. The Carthaginian laid a trap for his antagonist, into which the Roman fell with an almost ridiculous simplicity. The road southward led past Lake Trasumennus. Here it was narrow, the mountains approaching near to the water-side; a little further on there was an open space of some extent; after this again the mountains closed in again and made a narrow defile. These features are not visible to any one who approaches the place from the north; and the Consul seems to have taken no pains to acquaint himself with the road which he was following.

Hannibal barred the southern outlet with a strong force of his picked troops; his cavalry he put in ambush at the point of entrance; the high ground that bordered the road on the landward side he occupied with his slingers and light-armed troops. Flaminius reached the lake at sunset on the day of his breaking up his camp at Arretium, and bivouacked there for the night. Next day, at early dawn, he moved forward, again without reconnoitring, and reached the open space described before. A heavy morning mist hung over the country, and the Romans saw nothing but the road on which they were marching. Their first sight of the enemy was when they reached the defile where Hannibal himself was in position. Almost at the same moment the mist rolled away, and they saw that the mountain-sides on either hand were alive with enemies, and that their retreat was barred by the Carthaginian cavalry. At the same moment they found themselves attacked, before, says the historian, they could form their lines, or even draw their swords.

The result of this surprise was something like a panic. The march had been conducted with so much carelessness and disorder that the legions and even the maniples or companies were broken up. It was by the merest chance that a soldier found himself in his proper place, or ranged with his proper comrades. Some, it would seem, were actually without arms, for these were being carried in waggons, and the waggons could not be found when they were wanted. The mist, it must be remembered, though it had cleared away from the higher ground, still lay thick upon the lower ground, which was indeed very little raised above the level of the lake. So it came to pass that, as Livy puts it, the ear was of more service than the eye. The men rushed where they heard the groans of the wounded, the clash of sword upon armour, the cry of victory or defeat. The coward, flying in terror, found himself entangled in the mass of combatants; the brave man, eager to take his part in the struggle, might be irresistibly carried off by a crowd of fugitives.

After a while something of the habitual Roman courage reasserted itself. Every one could see for himself that the army was hemmed in. The mountains were on one side, the lake on the other; at either end of the road the passage was barred by serried lines of the enemy. If there was to be any deliverance it must come from their own strength and valour. Panic was succeeded by the courage of despair. Nothing could restore to the army its lost order, but it was at least determined to sell its existence dearly. And here, at least, Flaminius did his duty to the utmost. Incompetent as he was as a general, he was the bravest of the brave. "It is not by prayers to heaven," he cried, "that you will escape. Strength and courage, and these alone, will save you. The less your fear, the smaller the danger." The men answered to their leader's call. So fierce was the fight that the combatants were wholly unconscious of an earthquake which, at the very hour when the battle raged most fiercely, laid more than one city in ruins, changed the courses of rivers, and brought down huge masses of earth and rock from the mountains to the plains.

It was round the person of the Consul that the battle raged most fiercely. He was a conspicuous figure in the scarlet cloak which marked the officer in chief command, and in arms of unusual splendour. And as long as he was in the front the legions held their own. For three hours the issue seemed to be in suspense. But a general who exposes himself as recklessly as the Consul felt constrained to do can hardly hope to escape. And there were some in the hostile ranks who bore him a special grudge. Five years before Flaminius had carried on a campaign against the Insubrian Gauls, and had treated them, it would seem, with exceptional severity. An Insubrian trooper now recognised him. "This is the man," he cried to his comrades, "who slaughtered our countrymen, and laid waste our fields. I will offer him a sacrifice to the spirits of the dead." So saying, he set spurs to his horse and charged through the Roman line. The Consul's armour-bearer threw himself in the way, and was struck down. The Consul himself fell mortally wounded. A fierce struggle took place over his body, but the Roman veterans succeeded in rescuing it. But to an army that is fighting at a disadvantage the fall of its leader is often a disabling blow. So it was at Lake Trasumennus. The Roman army no longer held its ground. Frantic attempts were made to fly. Some tried to climb the mountain-side; others endeavoured to escape by wading out into the lake. Very few succeeded in either attempt. In the lake, especially, many perished. Those who attempted to swim were drowned sooner or later; those who made their way back to the shore were cut down by the enemy's horsemen, who rode out in the shallow and were ready to receive them. Fifteen thousand in all were slain; ten thousand contrived to escape. One body of six thousand, possibly a complete legion, succeeded in forcing its way through the defile occupied by Hannibal's troops. But its fate was only delayed for a time. It was without provisions, and without guides. When, the next day, Maharbal with the Carthaginian cavalry appeared, it surrendered. Hannibal's loss was fifteen hundred slain and a very considerable number of wounded. Livy gives these figures on the authority of a contemporary writer, Fabius Pictor.