History of the Peloponnesian War

A few of the Peloponnesian garrison at first stood their ground and defended themselves, some of them being killed, but most of them took to flight, being seized with panic, both because the enemy had attacked them at night, and also because they thought the Megarian traitors were fighting against them; and they supposed that all the Megarians had betrayed them.

For it so happened also that the Athenian herald, acting on his own responsibility, made a proclamation that any Megarian who so desired might espouse the cause of the Athenians. When the garrison heard this proclamation it no longer held out, but, verily believing that a concerted attack was being made upon them, fled to Nisaea.

And at daybreak, when the walls had already been taken and the Megarians in the city were in a tumult, those who had negotiated with the Athenians, and a large number besides who were privy to the plot, expressed the opinion that they ought to open the gates and go out to battle.