History of the Peloponnesian War

They carried on these negotiations through Eurymachus, the son of Leontiades, a very influential person at Thebes. For the Thebans, foreseeing that the war would take place, wished to surprise Plataea, which had always been at variance with them, while it was still time of peace, and the war had not openly broken out. And on this account, too, they entered the more easily without being observed, as no guard had been set before [the gates].