The Histories · 7.130

So the guides, when Xerxes asked whether there was any other possible outlet to the sea for the Peneios, said with exact knowledge of the truth: "O king, for this river there is no other outgoing which extends to the sea, but this alone; for all Thessaly is circled about with mountains as with a crown." To this Xerxes is said to have replied: "The Thessalians then are prudent men. This it appears was that which they desired to guard against in good time when they changed their counsel, reflecting on this especially besides other things, namely that they had a country which, it appears, is easy to conquer and may quickly be taken: for it would have been necessary only to let the river flow over their land by making an embankment to keep it from going through the narrow channel and so diverting the course by which now it flows, in order to put all Thessaly under water except the…

Herodotus of Halicarnassus · trans. G. C. Macaulay, 1890
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