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; each expecting a battle between the Athenians and the relieving army, and thinking it safer to see their friends victorious before declaring in their favour.
Unable to carry his point, Brasidas went back to the rest of the army. At daybreak the Boeotians joined him. Having determined to relieve Megara, whose danger they considered their own, even before hearing from Brasidas, they were already in full force at Plataea, when his messenger arrived to add spurs to their resolution; and they at once sent on to him two thousand two hundred heavy infantry, and six hundred horse, returning home with the main body. The whole army thus assembled numbered six thousand heavy infantry. The Athenian heavy infantry were drawn up by Nisaea and the sea; but the light troops being scattered over the plain were attacked by the Boeotian horse and driven to the sea, being taken entirely by surprise, as on previous occasions no relief had ever come to the Megarians from any quarter. Here the Boeotians were in their turn charged and engaged by the Athenian horse, and a cavalry action ensued which lasted a long time, and in which both parties claimed the victory. The Athenians killed and stripped the leader of the Boeotian horse and some few of his comrades who had charged right up to Nisaea, and remaining masters of the bodies gave them back under truce, and set up a trophy; but regarding the action as a whole the forces separated without either side having gained a decisive advantage, the Boeotians returning to their army and the Athenians to Nisaea.
After this Brasidas and the army came nearer to the sea and to Megara, and taking up a convenient position, remained quiet in order of battle, expecting to be attacked by the Athenians and knowing that the Megarians were waiting to see which would be the victor. This attitude seemed to present two advantages. Without taking the offensive or willingly provoking the hazards of a battle, they openly showed their readiness to fight, and thus without bearing the burden of the day would fairly reap its honours; while at the same time they effectually served their interests at Megara. For if they had failed to show themselves they would not have had a chance, but would have certainly been considered vanquished, and have lost the town. As it was, the Athenians might possibly not be inclined to accept their challenge, and their object would be attained without fighting. And so it turned out. The Athenians formed outside the long walls and, the enemy not attacking, there remained motionless;their generals having decided that the risk was too unequal. In fact most of their objects had been already attained; and they would have to begin a battle against superior numbers, and if victorious could only gain Megara, while a defeat would destroy the flower of their heavy soldiery. For the enemy it was different; as even the states actually represented in his army risked each only a part of its entire force, he might well be more audacious. Accordingly, after waiting for some time without either side attacking, the Athenians withdrew to Nisaea, and the Peloponnesians after them to the point from which they had set out. The friends of the Megarian exiles now threw aside their hesitation, and opened the gates to Brasidas and the commanders from the different states- looking upon him as the victor and upon the Athenians as having declined the battle- and receiving them into the town proceeded to discuss matters with them; the party in correspondence with the Athenians being paralysed by the turn things had taken.
Afterwards Brasidas let the allies go home, and himself went back to Corinth, to prepare for his expedition to Thrace, his original destination. The Athenians also returning home, the Megarians in the city most implicated in the Athenian negotiation, knowing that they had been detected, presently disappeared; while the rest conferred with the friends of the exiles, and restored the party at Pegae, after binding them under solemn oaths to take no vengeance for the past, and only to consult the real interests of the town. However, as soon as they were in office, they held a review of the heavy infantry, and separating the battalions, picked out about a hundred of their enemies, and of those who were thought to be most involved in the correspondence with the Athenians, brought them before the people, and compelling the vote to be given openly, had them condemned and executed, and established a close oligarchy in the town- a revolution which lasted a very long while, although effected by a very few partisans.