History of the Peloponnesian War

Now if they wished to take the sea-route and make their passage by way of the Crisaean Gulf, the Athenians were sure to take their fleet round the Peloponnesus and block their way; and to march over the Geranaean pass appeared to them hazardous, since the Athenians held Megara and Pegae. Besides, the Geranaean pass was not easy to traverse and was at all times guarded by the Athenians, and at this present time, as the Lacedaemonians perceived, they intended to block their way. So they decided to wait in Boeotia and consider how they might most safely cross over to the Peloponnesus.

To this course they were partly influenced by some Athenians, who were secretly inviting them into their country, in the hope of putting an end to the democracy and to the building of the long walls.

But the Athenians went out against the Lacedaemonians with their whole force and with one thousand Argives and contingents of the several allies, the whole body amounting to fourteen thousand men.

And they undertook the expedition against them because they believed that they were at a loss how to get through, and partly too on a suspicion of a plot to overthrow the democracy.