The great savior of Greece at this crisis was the Athenian, Themistocles. He foresaw the renewed efforts of the Persian king to destroy Greece, and realized also that the most vital point in the coming conflict would be the control of the sea. Accordingly he urged upon the Athenians the necessity of building a powerful fleet. In this policy he was aided by one of those futile wars so characteristic of Greek history, a war between Athens and the island of AEgina. In order to overcome the AEginetans, who had a large fleet, the Athenians were compelled to build a larger one, and by the time this purpose was accomplished rumors came that the Persian king was getting ready another invasion of Greece. _Campaign of Salamis_ The third attempt was undertaken ten years after the second, in the year 480, under Xerxes, the successor to Darius. This time the very immensity of the forces employed was to overcome all opposition and all misfortunes. An army, variously estimated at from one to five million men, crossed the Hellespont on a bridge of boats to invade the peninsula from the north, while a fleet of 1200 triremes was assembled to insure the command of the sea. Against the unlimited resources of the Persian empire and the unity of plan represented by Xerxes and his generals, the Greeks had little to offer. They possessed the two advantages of the defensive, knowledge of the terrain and interior lines,[1] but their resources were small and their spirit divided. Greece in those days was, as was later said of Italy, "merely a geographical expression." The various cities were mutually jealous and hostile, and it took a great common danger to bring them even into a semblance of cooperation. Even during this desperate crisis the cities of western Greece, counting themselves reasonably safe from invasion, declined to send a ship or a man for the common cause. [Footnote 1: "'Interior Lines' conveys the meaning that from a central position one can assemble more rapidly on either of two opposite fronts than the enemy can, and therefore utilize force more effectively." NAVAL STRATEGY, A. T. Mahan, p. 32.] [Illustration: ROUTE OF XERXES' FLEET TO BATTLE OF SALAMIS]