The Art of War by active 6th century B.C. Sunzi

though he be sheltered behind a high rampart and a deep ditch. All we need do is attack some other place that he will be obliged to relieve. [Tu Mu says: "If the enemy is the invading party, we can cut his line of communications and occupy the roads by which he will have to return; if we are the invaders, we may direct our attack against the sovereign himself." It is clear that Sun TzÅ­, unlike certain generals in the late Boer war, was no believer in frontal attacks.]