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

39. If the enemy’s troops march up angrily and remain facing ours for a

long time without either joining battle or taking themselves off again, the situation is one that demands great vigilance and circumspection. [Ts’ao Kung says a manœuver of this sort may be only a ruse to gain time for an unexpected flank attack or the laying of an ambush.]