Plutarch, Sertorius
Bernadotte Perrin, Ed.

10. As he was deliberating whither to turn his efforts next, the Lusitanians sent ambassadors and invited him to be their leader. They were altogether lacking in a commander of great reputation and experience as they faced the terror of the Roman arms, and they entrusted themselves to him, and to him alone, when they learned about his character from those who had been with him. [2] And it is said that Sertorius was no easy victim either of pleasure or of fear, but that he was naturally unterrified in the face of danger, and bore prosperity with moderation; in straightforward fighting he was as bold as any commander of his time, while in all military activities demanding stealth and the power to seize an advantage in securing strong positions or in crossing rivers, where speed, deceit, and, if necessary, falsehood are required, he was an expert of the highest ability. [3] Moreover, while he showed himself generous in rewarding deeds of valour, he used moderation in punishing transgressions. And yet, in the last part of his life, the savage and vindictive treatment which he bestowed upon his hostages¹ would seem to show that his mildness was not natural to him, but was worn as a garment, from calculation, as necessity required. [4] In my opinion, however, a virtue that is sincere and based upon reason can never by any fortune be converted into its opposite, although it is true that excellent principles and natures, when impaired by great and undeserved calamities, may possibly change their character as the guiding genius changes. And this, I think, was the case with Sertorius when fortune at last began to forsake him; as his cause grew hopeless he became harsh toward those who did him wrong.