C. Julius Caesar, Gallic War

The next day (as there remained in all only two day's space [to the time] when he must serve out the corn to his army, and as he was not more than eighteen miles from Bibracte , by far the largest and best-stored town of the Aedui), he thought that he ought to provide for a supply of corn; and diverted his march from the Helvetii, and advanced rapidly to Bibracte . This circumstance is reported to the enemy by some deserters from Lucius Aemilius, a captain, of the Gallic horse. The Helvetii, either because they thought that the Romans, struck with terror, were retreating from them, the more so, as the day before, though they had seized on the higher grounds, they had not joined battle or because they flattered themselves that they might be cut of from the provisions, altering their plan and changing their route, began to pursue, and to annoy our men in the rear.