History of the Peloponnesian War

It had been concerted between them that the Athenians should rush in, the moment that the gates were opened, while the conspirators were to be distinguished from the rest by being anointed with oil, and so to avoid being hurt. They could open the gates with more security, as four thousand Athenian heavy infantry from Eleusis, and six hundred horse, had marched all night, according to agreement, and were now close at hand.

The conspirators were all ready anointed and at their posts by the gates when one of their accomplices denounced the plot to the opposite party, who gathered together and came in a body, and roundly said that they must not march out—a thing they had never yet ventured on even when in greater force than at present—or wantonly compromise the safety of the town, and that if what they said was not attended to, the battle would have to be fought in Megara. For the rest, they gave no signs of their knowledge of the intrigue, but stoutly maintained that their advice was the best, and meanwhile kept close by and watched the gates, making it impossible for the conspirators to effect their purpose.

The Athenian generals seeing that some obstacle had arisen, and that the capture of the town by force was no longer practicable, at once proceeded to invest Nisaea, thinking that if they could take it before relief arrived, the surrender of Megara would soon follow.