History of the Peloponnesian War

This was arranged through Eurymachus, son of Leontiades, a person of great influence at Thebes. For Plataea had always been at variance with Thebes; and the latter, foreseeing that war was at hand, wished to surprise her old enemy in time of peace, before hostilities had actually broken out. Indeed this was how they got in so easily without being observed, as no guard had been posted.