History of the Peloponnesian War

Now by sea, if they should wish to cross over the Crissaean Gulf, the Athenians were ready to stop them, having sailed round with a fleet: while the march over Geranea did not appear safe for them as the Athenians were in possession of Megara and Pega. For Geranea was both [naturally] difficult to cross, and was continually guarded by the Athenians : and at that time they knew they were going to stop them that way, as well [as by sea].

So they determined to wait in Boeotia, and see in what way they might march across most safely. They were also in some measure urged to this in secret by certain of the Athenians, who hoped to put a stop to the democracy, and to the long walls that were building.