History of the Peloponnesian War
Thucydides
Thucydides. History of the Peloponnesian War. Crawley, Richard, translator. London and Toronto: J. M. Dent and Sons Ltd.; New York: E. P. Dutton and Co., 1914.
and by the afternoon of the next the wall was all but completed, when the garrison in Nisaea, alarmed by the absolute want of provisions, which they used to take in for the day from the upper town, not anticipating any speedy relief from the Peloponnesians, and supposing Megara to be hostile, capitulated to the Athenians on condition that they should give up their arms, and should each be ransomed for a stipulated sum; their Lacedaemonian commander, and any others of his countrymen in the place, being left to the discretion of the Athenians.
On these conditions they surrendered and came out, and the Athenians broke down the long walls at their point of junction with Megara, took possession of Nisaea, and went on with their other preparations.
Just at this time the Lacedaemonian Brasidas, son of Tellis, happened to be in the neighbourhood of Sicyon and Corinth, getting ready an army for Thrace. As soon as he heard of the capture of the walls, fearing for the Peloponnesians in Nisaea and the safety of Megara, he sent to the Boeotians to meet him as quickly as possible at Tripodiscus, a village so called of the Megarid, under Mount Geraneia, and went himself, with two thousand seven hundred Corinthian heavy infantry, four hundred Phliasians, six hundred Sicyonians, and such troops of his own as he had already levied, expecting to find Nisaea not yet taken.
Hearing of its fall (he had marched out by night to Tripodiscus), he took three hundred picked men from the army, without waiting till his coming should be known, and came up to Megara unobserved by the Athenians, who were down by the sea, ostensibly, and really if possible, to attempt Nisaea, but above all to get into Megara and secure the town. He accordingly invited the townspeople to admit his party, saying that he had hopes of recovering Nisaea.
However, one of the Megarian factions feared that he might expel them and restore the exiles; the other that the commons, apprehensive of this very danger, might set upon them, and the city be thus destroyed by a battle within its gates under the eyes of the ambushed Athenians. He was accordingly refused admittance, both parties electing to remain quiet and await the event;