This question would, of course, depend very much upon the disposition of Thessaly herself. The government of that country, understanding the critical situation in which they were placed, had not waited for the Athenians and Spartans to send embassadors to them, but, at a very early period of the war--before, in fact, Xerxes had yet crossed the Hellespont, had sent messengers to Athens to concert some plan of action. These messengers were to say to the Athenians that the government of Thessaly were expecting every day to receive a summons from Xerxes, and that they must speedily decide what they were to do; that they themselves were very unwilling to submit to him, but they could not undertake to make a stand against his immense host alone; that the southern Greeks might include Thessaly in their plan of defense, or exclude it, just as they thought best. If they decided to include it, then they must make a stand at the Olympic Straits, that is, at the pass between Olympus and Ossa; and to do that, it would be necessary to send a strong force immediately to take possession of the pass. If, on the contrary, they decided _not_ to defend Thessaly, then the pass of Thermopylæ would be the point at which they must make their stand, and in that case Thessaly must be at liberty to submit on the first Persian summons.
The Greeks, after consultation on the subject, decided that it would be best for them to defend Thessaly, and to take their stand, accordingly, at the Straits of Olympus. They immediately put a large force on board their fleet, armed and equipped for the expedition. This was at the time when Xerxes was just about crossing the Hellespont. The fleet sailed from the port of Athens, passed up through the narrow strait called Euripus, lying between the island of Euboea and the main land, and finally landed at a favorable point of disembarkation, south of Thessaly. From this point the forces marched to the northward until they reached the Peneus, and then established themselves at the narrowest part of the passage between the mountains, strengthened their position there as much as possible, and awaited the coming of the enemy. The amount of the force was ten thousand men.