History of the Peloponnesian War

When Amphipolis was in the enemy's hands, the- Athenians were reduced to great fear, especially because the town was of service to them by supplying timber for ship building, and in point of payment of revenue; and because, though as far as the Strymon the Lacedaemonians had a passage open to them for reaching the allies of Athens, if the Thessalians allowed them to go through their country, yet so long as they were not masters of the bridge, they could have gone no further; as on the inland side a large lake, formed by the river, spread for a great distance, while in the neighbourhood of Eion they were watched by cruisers:

but now the passage was considered to have been rendered easy. They were also afraid that their allies would revolt.