Einhard

“In the midst of this vigorous and almost uninterrupted struggle with the Saxons, he covered the frontier by garrisons at the proper points, and marched over the Pyrenees into Spain at the head of all the forces that he could muster. All the towns and castles that he attacked surrendered. and up to the time of his homeward march he sustained no loss whatever; but on his return through the Pyrenees he had cause to rue the treachery of the Gascons. That region is well adapted for ambuscades by reason of the thick forests that cover it; and as the army was advancing in the long line of march necessitated by the narrowness of the road, the Gascons, who lay in ambush on the top of a very high mountain, attacked the rear of the baggage train and the rear guard in charge of it, and hurled them down to the very bottom of the valley [at Roncevalles, later celebrated in the Song of Roland].”
― Einhard, The Life of Charlemagne