In a war between Sparta and Argos, King Cleomenes was in charge of the Spartan troops. Sparta had such a reputation for military prowess and trickery that the Argives, for fear of being caught off-guard, matched every Spartan order. If Cleomenes ordered his troops to arm themselves, the Argives did the same. If he ordered his troops to rest, the Argives did the same. Cleomenes therefore gave secret orders to his troops that the next time he ordered a retreat, they should instead advance. When he gave that order, the Argives sound found themselves being pressed upon while unarmed and unprepared, and Cleomenes thereby gained an easy victory.
It’s not well-known, but despite the popular conception of Spartans as strong, honorable warriors, they actually preferred victory by deceit. If a victory were won by force, a rooster were sacrificed. If by deceit, a bull. They were excellent warriors, but they would rather win without battle.