Vegetius Quote

It is the nature of war that what is beneficial to you is detrimental to the enemy and what is of service to him always hurts you. It is therefore a maxim never to do, or to omit doing, anything as a consequence of his actions, but to consult invariably your own interest only. And you depart from this interest whenever you imitate such measures as he pursues for his benefit. For the same reason, it would be wrong for him to follow such steps as you take for your advantage.

De Re Militari (also Epitoma Rei Militaris) - Book III, "Dispositions for Action"