Vegetius Quote

It is much better to overcome the enemy by famine, surprise or terror than by general actions, for in the latter instance fortune has often a greater share than valour. Those designs are best which the enemy are entirely ignorant of till the moment of execution. Opportunity in war is often more to be depended on than courage.

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