History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
An indecisive battle was fought with the Corinthians, whereupon they separated, each side thinking they had not got the worst of it in the action.
And the Athenians, who had in fact got rather the better of it, when the Corinthians withdrew, set up a trophy; but the Corinthians, being reproached by the older men in their city, made their preparations and about twelve days later came back and set up for themselves a rival trophy, as though they had won. Hereupon the Athenians made a sally from Megara, slew those who were setting up the trophy, and joining battle with the rest defeated them.
The vanquished party now retreated, and a not inconsiderable portion of them, being hard pressed, missed their way and rushed into a piece of land belonging to some private person, which was enclosed by a great ditch and had no exit.
And when the Athenians perceived this, they shut them in by barring the entrance with hoplites, and stationing light-armed troops all round stoned all who had entered. And this was a great calamity to the Corinthians; the main body of their army, however, returned home.
About this period[*](457 B.C.) the Athenians began to build their long walls to the sea, one to Phalerum, the other to the Peiraeus. And the Phocians made an expedition against the land of the Dorians, the mother-country of the Lacedaemonians, namely the towns of Boeum, Citinium, and Erineum, one of which they captured;
whereupon the Lacedaemonians, under the lead of Nicomedes son of Cleombrotus, acting for King Pleistoanax son of Pausanias, who was still a minor, sent to the aid of the Dorians a force of fifteen hundred hoplites of their own and ten thousand of their allies, and after they had forced the Phocians to make terms and restore the city they began their return homeward.
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.