Titus Livius (Livy), The History of Rome, Book 25
D. Spillan, A.M., M.D., Cyrus Evans, Ed.

33. Hasdrubal perceiving that there were but few Roman troops in the camp, and that their whole dependence was on the Celtiberian auxiliaries; [2] and having had experience of the perfidy of the barbarian nations in general, and particularly of all those nations among which he had served for so many years; [3] as there was every facility of intercourse, for both camps were full of Spaniards, by secret conferences with the chiefs of the Celtiberians, he agreed with them, for a large consideration, to take their forces away. [4] Nor did they conceive it to be any great crime; for the object was not that they should turn their arms against the Romans, while the reward which they were to receive to abstain from the war was large enough to remunerate them for their service in it. At the same time the mere rest from labour, the return to their homes, with the pleasure of seeing their friends and property, were pleasing to the generality. [5] Accordingly, the multitude were prevailed upon as easily as their leaders. They [p. 1004]had, moreover, nothing to fear from the Romans, in consequence of the smallness of their numbers, should they endeavour to detain them by force. [6] It will indeed be the duty of all Roman generals to take care, and the instances here recorded should be considered as strong arguments, never to place so much confidence in foreign auxiliaries, as not to retain in their camps a preponderance of their own strength, and of that force which is properly their own. [7] The Celtiberians, suddenly taking up their standards, marched away, replying only to the Romans, who asked the cause of their departure and entreated them to stay, that they were called away by a war at home. [8] Scipio seeing that his allies could be detained neither by prayers nor force, and that he was neither a match for his enemy without them, nor could again effect a junction with his brother, no other course which promised safety offering itself, resolved to retire [9??] as far as possible, carefully using every caution not to encounter the enemy any where on level ground. On his departing, the enemy, crossing the river, pursued him almost in his footsteps.