And, in truth, it was through this passage that the Persian army ultimately marched.
The Greeks, therefore, broke up the camp and returned to the Isthmus.
The Thessalians, thus abandoned, instantly treated with the invader, and became among the stanchest allies of Xerxes.
It was now finally agreed in the Isthmian congress, that the most advisable plan would be to defend the pass of Thermopylae, as being both nearer and narrower than that of Thessaly.

The fleet they resolved to send to Artemisium, on the coast of Histiaeotis, a place sufficiently neighbouring Thermopylae to allow of easy communication.
Never, perhaps, have the Greeks shown more military skill than in the choice of these stations.

But one pass in those mountainous districts permitted the descent of the Persian army from Thessaly, bounded to the west by steep and inaccessible cliffs, extending as far as Mount Oeta; to the east by shoals and the neighbouring sea.

This defile received its name Thermopylae, or Hot Gates, from the hot-springs which rose near the base of the mountain.

In remote times the pastoral Phocians had fortified the place against the incursions of the Thessalians, and the decayed remains of the wall and gates of their ancient garrison were still existent in the middle of the pass; while, by marsh and morass, to render the place yet more impassable, they had suffered the hot-springs to empty themselves along the plain, on the Thessalian side, and the quagmire was still sodden and unsteady.