History of the Peloponnesian War

For there were both Argilians that dwelt in the same city (now Argilus is a colony of the Andrians), and others, that contrived this, induced thereunto some by Perdiccas and some by the Chalcideans.

But above all the Argilians, being of a city near unto it, and ever suspected by the Athenians, and secret enemies to the place, as soon as opportunity was offered and Brasidas arrived (who had also long before dealt underhand with as many of them as dwelt in Amphipolis to betray it), both received him into their own city, and revolting from the Athenians, brought the army forward the same night as far as to the bridge of the river.

The town stood not close to the river, nor was there a fort at the bridge then as there is now; but they kept it only with a small guard of soldiers. Having easily forced this guard, both in respect of the treason and of the weather, and of his own unexpected approach, he passed the bridge and was presently master of whatsoever the Amphipolitans had that dwelt without.

Having thus suddenly passed the bridge, and many of those without being slain, and some fled into the city, the Amphipolitans were in very great confusion at it; and the rather because they were jealous one of another.

And it is said that if Brasidas had not sent out his army to take booty, but had marched presently to the city, he had in all likelihood taken it then. But so it was that he pitched there and fell upon those without;