 When the Athenian emissaries delivered an ultimatum to the Spartans the next day, they were amazed to hear that a task force was in fact already "en route"; the Spartan army was marching to meet the Persians.When Mardonius learned of the Spartan force, he completed the destruction of Athens, tearing down whatever was left standing.He then retreated towards Thebes, hoping to lure the Greek army into territory that would be suitable for the Persian cavalry.