Half way along these hills a stream runs down a valley into the lake, and in the valley, completely hidden from the sight of an enemy approaching, Hannibal placed the Numidian cavalry and the Gaulish infantry.  Among some woods clothing the lower slope of the hills facing the lake he placed his light troops, while the Spanish and African infantry and the Gaulish cavalry were similarly hidden on the outer slopes of the hill in readiness to close in on the rear of the Romans when they had entered on the road between the hills and the lake.

No better position could have been chosen for a surprise.  When once the Romans had entered the path between the hills and the lake there was no escape for them.  They were shut up between the wood clad hills swarming with the Carthaginian light troops and the lake, while the heavy infantry and cavalry of Hannibal were ready to fall on them front and rear.

When Flaminius arrived at Cortona late at night he heard of the ravages and executions committed by the Carthaginians, as they had passed through early in the morning, and resolved to press forward at daybreak in hopes of finding some opportunity for falling upon and punishing them.  When day broke it seemed favourable to his design, for a thick mist was rising from the lake and marshes.  This, he thought, would conceal his advance from the Carthaginians, while, as the high ground ahead rose above the mist, he would be enabled to see their position.  He pushed forward then rapidly, thinking that he should be able to overtake the rear of the Carthaginian army as it moved slowly along encumbered with its plunder.

As he neared the entrance to the pass he caught sight of the heavy armed Carthaginians on the distant hill above the level of the mist, and believing that his own movements were hidden from the enemy, pushed forward as fast as the infantry could march.  But the moment the rear of his column had entered the narrow flat between the foot of the hills and the lake, the Numidians quietly moved down and closed the pass behind them, while Hannibal with his heavy infantry descended from the farther hill to confront him.  When all was ready he gave the signal, and at once in front, on their right flank, and on their rear the Carthaginians fell upon them.

The light troops heralded their attack by rolling a vast quantity of rocks down the hill on the long column, and then, pressing down through the woods, poured their arrows and javelins into the struggling mass.

Taken wholly by surprise, unable to advance or retreat, desperate at finding themselves thus caught in a trap, the Romans fought bravely but in vain.  An earthquake shook the ground on which the terrible fight was going on; but not for a moment did it interrupt the struggle.  For three hours the Romans, although suffering terribly, still fought on; then Flaminius was killed, and from that time they thought only of escape.  But this was next to impossible.  Six thousand only cut their way out.  Fifteen thousand fell, and nine thousand were taken prisoners.