The position had been considered, long before Xerxes's day, a very important one in a military point of view, as it was upon the frontier between two Greek states that were frequently at war. One of these states, of course, was Thessaly. The other was Phocis, which lay south of Thessaly. The general boundary between these two states was mountainous, and impassable for troops, so that each could invade the territories of the other only by passing round between the mountains and the shore at Thermopylae. The Phocaeans, in order to keep the Thessalians out, had, in former times, built a wall across the way, and put up gates there, which they strongly fortified. In order still further to increase the difficulty of forcing a passage, they conducted the water of the warm springs over the ground without the wall, in such a way as to make the surface continually wet and miry. The old wall had now fallen to ruins, but the miry ground remained. The place was solitary and desolate, and overgrown with a confused and wild vegetation. On one side the view extended far and wide over the sea, with the highlands of Euboea in the distance, and on the other dark and inaccessible mountains rose, covered with forests, indented with mysterious and unexplored ravines, and frowning in a wild and gloomy majesty over the narrow passway which crept along the shore below. The Greeks, when they retired from Thessaly, fell back upon Thermopylae, and established themselves there. They had a force variously estimated, from three to four thousand men. These were from the different states of Greece, some within and some without the Peloponnesus--a few hundred men only being furnished, in general, from each state or kingdom. Each of these bodies of troops had its own officers, though there was one general-in-chief, who commanded the whole. This was Leonidas the Spartan. He had brought with him three hundred Spartans, as the quota furnished by that city. These men he had specially selected himself, one by one, from among the troops of the city, as men on whom he could rely. It will be seen from the map that Thermopylae is at some distance from the Isthmus of Corinth, and that of the states which would be protected by making a stand at the pass, some were without the isthmus and some within. These states, in sending each a few hundred men only to Thermopylae, did not consider that they were making their full contribution to the army, but only sending forward for the emergency those that could be dispatched at once; and they were all making arrangements to supply more troops as soon as they could be raised and equipped for the service. In the mean time, however, Xerxes and his immense hordes came on faster than they had expected, and the news at length came to Leonidas, in the pass, that the Persians, with one or two millions of men, were at hand, while he had only three or four thousand at Thermopylae to oppose them. The question arose, What was to be done?