History of the Peloponnesian War
Thucydides
Thucydides, Vol. 1-4. Smith, Charles Foster, translator. London and Cambridge, MA: Heinemann and Harvard University Press, 1919-1923.
Brasidas easily forced the small guard stationed at the bridge, partly because there was treachery, partly because he had fallen upon them in stormy weather and unexpectedly; and as soon as he had crossed the bridge he was at once master of the property of the Amphipolitans outside the walls; for they had houses all over the neighbourhood.
His crossing had surprised the people inside the city, and of those outside many were captured, while others took refuge within the walls; hence the Amphipolitans were thrown into great confusion, especially as they were suspicious of each other.
Indeed the general impression was, it is said, that if Brasidas, instead of turning to pillage with his army, had decided to march straight against the city, he could have taken it.
But as it was, when he had overrun the country outside and found that none of his plans were being carried out by his friends within the city, he merely settled his army in camp and kept quiet.