But the most important of all these negotiations of the Athenians and Spartans with the neighboring states were those opened with Thessaly. Thessaly was a kingdom in the northern part of Greece. It was, therefore, the territory which the Persian armies would first enter, on turning the northwestern corner of the Ægean Sea. There were, moreover, certain points in its geographical position, and in the physical conformation of the country, that gave it a peculiar importance in respect to the approaching conflict.
By referring to the map placed at the commencement of the fifth chapter, it will be seen that Thessaly was a vast valley, surrounded on all sides by mountainous land, and drained by the River Peneus and its branches. The Peneus flows eastwardly to the Ægean Sea, and escapes from the great valley through a narrow and romantic pass lying between the Mountains Olympus and Ossa. This pass was called in ancient times the Olympic Straits, and a part of it formed a romantic and beautiful glen called the Vale of Tempe. There was a road through this pass, which was the only access by which Thessaly could be entered from the eastward.
To the south of the Vale of Tempe, the mountains, as will appear from the map, crowded so hard upon the sea as not to allow any passage to the eastward of them. The natural route of Xerxes, therefore, in descending into Greece, would be to come down along the coast until he reached the mouth of the Peneus, and then, following the river up through the Vale of Tempe into Thessaly, to pass down toward the Peloponnesus on the western side of Ossa and Pelion, and of the other mountains near the sea. If he could get through the Olympic Straits and the Vale of Tempe, the way would be open and unobstructed until he should reach the southern frontier of Thessaly, where there was another narrow pass leading from Thessaly into Greece. This last defile was close to the sea, and was called the Straits of Thermopylæ.
Thus Xerxes and his hosts, in continuing their march to the southward, must necessarily traverse Thessaly, and in doing so they would have two narrow and dangerous defiles to pass--one at Mount Olympus, to get into the country, and the other at Thermopylæ, to get out of it. It consequently became a point of great importance to the Greeks to determine at which of these two passes they should make their stand against the torrent which was coming down upon them.