I'm confident I read this somewhere, but I can't seem to find it in any of my books or on the internet. The details that I remember are of an army setting up camp outside a fortress or stronghold. The general of the outside army instead of attacking right away ran his army through battle formations and practice in front of the enemy and then would have his men charge the gate, only to divert at the last second. He continually did this and lured the army within the stronghold into complacency where they eventually viewed the army as less of a threat and would take longer to man their positions. Eventually, one day, the outside army charged like any other day, but this time did not stop, taking the stronghold and exploiting the others complacency. Any ideas or suggestions would help. Thank you.
You may be thinking of a better-known battle, but a similar tactic is described in Frontinus' Stratagems:
When Domitius Calvinus was besieging Lueria, a town of the Ligurians, protected not only by its location and siege-works, but also by the superiority of its defenders, he instituted the practice of marching frequently round the walls with all his forces, and then marching back to camp. When the townspeople had been induced by this routine to believe that the Roman commander did this for the purpose of drill, and consequently took no precautions against his efforts, he transformed this practice of parading into a sudden attack, and gaining possession of the walls, forced the inhabitants to surrender.
Not a siege, but a pitched battle. This was the Battle of Illipa, between Scipio Africanus (the one who beat Hannibal at Zama) and Hasdrubal Gisco in Hispania. This was early in Scipio's career, before he fought Hannibal at Zama. Initially Scipio had defeated Hasdrubal Barca at the Battle of Baecula, and the other Carthaginian generals in the area grouped up, under Hasdrubal Gisco. They attacked the Roman camp with cavalry but were repelled.
Scipio then each day for a few days would march his army out in front of the Carthaginian camp after they had marched out and put his troops in a certain formation, having his legionaries in the middle and his Iberian allies and light troops on the flanks.
After doing this for several days, he marched out before dawn with his troops in a different formation, the the legionaries on the flanks, similar to Hannibal's tactic at the Battle of Cannae. and attacked the Carthaginians. The Carthaginians were completely unprepared and marched out to fight the formation that Scipio had been fielding for the past few days. Their flanks crumpled quickly, however, as the strongest troops were in the middle of their formation to fight the legionaries that had previously been positioned there. Scipio then ordered his flanks to press in, and Hasdrubal's elephants under Mago Barca stampeded. Then to strike the hammer on the anvil he ordered his Iberians in the center to attack the Carthaginians.
The only thing that stopped the Romans from completely obliterating the Carthaginians was a thunderstorm that sprang up and stopped the battle.
Later however, when Hasdrubal tried to retreat, Scipio ordered a pursuit which when it caught up, according to wikipedia, left him with only 6,000 troops of his original 50,000-70,000, depending on which source you're looking at.
Thank you all for the responses, I believe it was Frontinus that I had read. Thanks to everyone else that got back to me as well, I didn't realize how a valuable a resource this site could be.