History of the Peloponnesian War
Thucydides. The Peloponnesian War. London, J. M. Dent; New York, E. P. Dutton. 1910.
After hearing them, the Lacedaemonians determined to send out the colony, wishing to assist the Trachinians and Dorians, and also because they thought that the proposed town would lie conveniently for the purposes of the war against the Athenians. A fleet might be got ready there against Euboea, with the advantage of a short passage to the island; and the town would also be useful as a station on the road to Thrace. In short, everything made the Lacedaemonians eager to found the place.
After first consulting the god at Delphi and receiving a favorable answer, they sent off the colonists, Spartans and Perioeci, inviting also any of the rest of the Hellenes who might wish to accompany them, except Ionians, Achaeans, and certain other nationalities; three Lacedaemonians leading as founders of the colony, Leon, Alcidas, and Damagon.
The settlement effected, they fortified anew the city, now called Heraclea, distant about four miles and a half from Thermopylae and two miles and a quarter from the sea, and commenced building docks, closing the side towards Thermopylae just by the pass itself, in order that they might be easily defended.
The foundation of this town, evidently meant to annoy Euboea (the passage across to Cenaeum in that island being a short one), at first caused some alarm at Athens, which the event however did nothing to justify, the town never giving them any trouble.
The reason of this was as follows. The Thessalians, who were sovereign in those parts, and whose territory was menaced by its foundation, were afraid that it might prove a very powerful neighbor, and accordingly continually harassed and made war upon the new settlers, until they at last wore them out in spite of their originally considerable numbers, people flocking from all quarters to a place founded by the Lacedaemonians, and thus thought secure of prosperity. On the other hand the Lacedaemonians themselves, in the persons of their governors, did their full share towards ruining its prosperity and reducing its population, as they frightened away the greater part of the inhabitants by governing harshly and in some cases not fairly, and thus made it easier for their neighbors to prevail against them.