The secret of this was their general extraordinary success, which made them confuse their strength with their hopes.
The same summer the Megarians in the city, pressed by the hostilities of the Athenians, who invaded their country twice every year with all their forces, and harassed by the incursions of their own exiles at Pegae, who had been expelled in a revolution by the popular party, began to ask each other whether it would not be better to receive back their exiles, and free the town from one of its two scourges.

The friends of the emigrants, perceiving the agitation, now more openly than before demanded the adoption of this proposition; and the leaders of the commons, seeing that the sufferings of the times had tired out the constancy of their supporters, entered in their alarm into correspondence with the Athenian generals, Hippocrates, son of Ariphron, and Demosthenes, son of Alcisthenes, and resolved to betray the town, thinking this less dangerous to themselves than the return of the party which they had banished.

It was accordingly arranged that the Athenians should first take the long walls extending for nearly a mile from the city to the port of Nisaea, to prevent the Peloponnesians coming to the rescue from that place, where they formed the sole garrison to secure the fidelity of Megara; and that after this the attempt should be made to put into their hands the upper town, which it was thought would then come over with less difficulty.
The Athenians, after plans had been arranged between themselves and their correspondents both as to words and actions, sailed by night to Minoa, the island off Megara, with six hundred heavy infantry under the command of Hippocrates, and took post in a quarry not far off, out of which bricks used to be taken for the walls; while Demosthenes, the other commander, with a detachment of Plataean light troops and another of Peripoli, placed himself in ambush in the precinct of Enyalius, which was still nearer.

No one knew of it, except those whose business it was to know that night.

A little before daybreak, the traitors in Megara began to act.