Titus Livius (Livy), The History of Rome, Book 22
Benjamin Oliver Foster, Ph.D., Ed.

4. Hannibal laid waste the land between the city of Cortona and Lake Trasumennus with every circumstance of cruelty known to war, in order the more to whet his enemy's anger and prompt him to avenge the sufferings of his allies. [2] And now he had reached a spot designed by nature for an ambuscade, where Trasumennus approaches closest to the mountains of Cortona. Between them is nothing but a very narrow track, as though room had been left expressly for this purpose; the ground then widens into a little plain; beyond this the hills rise steeply. [3] At this point he laid out a camp in the open, for himself and his African and Spanish troops only; the Baliares and the rest of his light-armed forces he led round behind the mountains; the cavalry he stationed near the entrance to the defile, where some hillocks formed a convenient screen for them, so that when the Romans should have entered the pass, they might block the road, and trap the entire army between the lake and the mountains.

Flaminius had reached the lake at sunset; the next morning, without reconnoitring, and scarcely waiting for broad daylight, he passed through the [p. 215]defile. [4] As the column began to spread out on the¹ more open ground, they caught sight of those enemies only who were right in front of them; the ambush in their rear and that above them they failed to perceive. [5] The Phoenician had now gained his object, the Romans were hemmed in between the mountains and the lake and their escape cut off by his own troops, when he made the signal for all his forces to attack at once. [6] As they charged down, each at the nearest point, their onset was all the more sudden and unforeseen inasmuch as the mist from the lake lay less thickly on the heights than on the plain, and the attacking columns had been clearly visible to one another from the various hills and had therefore delivered their charge at more nearly the same instant. [7] From the shouting that arose on every side the Romans learned, before they could clearly see, that they were surrounded; and they were already engaged on their front and flank before they could properly form up or get out their arms and draw their swords.