Sertorius (18.4-18.5)

When Sertorius heard of this, he gave a laugh, and said that to Sulla’s pupil (for thus he was wont to style Pompey in jest) he himself would give a lesson, namely, that a general must look behind him rather than in front of him. As he said this, he pointed out to his beleaguered troops six thousand men-at-arms whom he had left behind at their former camp, from which he had sallied forth to seize the hill; these, in case Pompey moved against the occupants of the hill, were to fall upon his rear.

Pompey also became aware of this all too late, and did not venture to attack Sertorius for fear of being surrounded, but line was ashamed to go away and leave the people of the city in their peril, and so was compelled to sit there quietly and see them ruined; for the Barbarians gave up all hope and surrendered to Sertorius.