Thucydides, History of the Peloponnesian War
Benjamin Jowett, Ed.

103. Against Amphipolis Brasidas now led his army. Starting from Arnae in Chalcidicè, towards¹ evening he reached Aulon and Bromiscus at the point where the lake Bolbè flows into the sea; having there supped, he marched on during the night. The weather was wintry and somewhat snowy; [2] and so he pushed on all the quicker; he was hoping that his approach might be known at Amphipolis only to those who were in the secret. [3] There dwelt in the place settlers from Argilus, a town which was originally colonised from Andros; these and others aided in the attempt, instigated some by Perdiccas, others by the Chalcidians. [4] The town of Argilus is not far off, and the inhabitants were always suspected by the Athenians, and were always conspiring against Amphipolis. For some time past, ever since the arrival of Brasidas had given them an opportunity, they had been concerting measures with their countrymen inside the walls for the surrender of the city. They now revolted from the Athenians on that very night, and received him into their town, and before dawn² they conducted the army to the bridge over the river, which is at some distance from the town. [5] At that time no walls had been built down to the river, as they have since been; a small guard was posted there. Brasidas easily overcame the guard, owing partly to the plot within the walls, partly to the severity of the weather and the suddenness of his attack; he then crossed the bridge, and at once was master of all the possessions of the Amphipolitans outside the walls. For they lived scattered about in the country.