The defeat of the Persians at Marathon served only to increase the resentment of Darius. He now resolved to collect the whole forces of his empire, and to lead them in person against Athens. For three years busy preparations were made throughout his vast dominions. In the fourth year his attention was distracted by a revolt of the Egyptians; and before he could reduce them to subjection he was surprised by death, after a reign of 37 years (B.C. 485). Xerxes, the son and successor of Darius, had received the education of an eastern despot, and been surrounded with slaves from his cradle. In person he was the tallest and handsomest man amidst the vast hosts which he led against Greece; but there was nothing in his mind to correspond to this fair exterior. His character was marked by faint-hearted timidity and childish vanity. Xerxes had not inherited his father's animosity against Greece; but he was surrounded by men who urged him to continue the enterprise. Foremost among these was Mardonius, who was eager to retrieve his reputation, and to obtain the conquered country as a satrapy for himself after subduing Egypt (B.C. 484), Xerxes began to make preparations for the invasion of Greece. For four years the din of preparation sounded throughout Asia. Troops were collected from every quarter of the Persian empire, and were ordered to assemble in Cappadocia. As many as forty-six different nations composed the land-force, of various complexions, languages, dresses, and arms. Meantime Xerxes ordered a bridge to be thrown across the Hellespont, that his army might march from Asia into Europe: and he likewise gave directions that a canal should be cut through the isthmus of Mount Athos, in order to avoid the necessity of doubling this dangerous promontory, where the fleet of Mardonius had suffered shipwreck. The making of this canal, which was about a mile and a half long employed a number of men for three years.

In the spring of B.C. 480 Xerxes set out from Sardis with his vast host. Upon reaching Abydos on the Hellespont the army crossed over to Europe by the bridge of boats. Xerxes surveyed the scene from a marble throne. His heart swelled within him at the sight of such a vast assemblage of human beings; but his feelings of pride and pleasure soon gave way to sadness, and he burst into tears at the reflection that in a hundred years not one of them would be alive. Xerxes continued his march through Europe along the coast of Thrace. Upon arriving at the spacious plain of Doriscus, which is traversed by the river Hebrus, he resolved to number his forces. He found that the whole armament, both military and naval, consisted of 2,317,610 men. In his march from Doriscus to Thermopylae he received a still further accession of strength; and accordingly when he reached Thermopylae the land and sea forces amounted to 2,641,610 fighting men. The attendants are said to have been more in number than the fighting men; but if they were only equal, the number of persons who accompanied Xerxes to Thermopylae reaches the astounding figure of 5,283,220! The number is quite incredible; but though the exact number of the invading army cannot be determined, we may safely conclude, from all the circumstances of the case, that it was the largest ever assembled at any period of history.

From Doriscus Xerxes his march along the coast through Thrace and Macedonia. The principal cities through which he passed had to furnish a day's meal for the immense host, and for this purpose had made preparations many months before-hand. The cost of feeding such a multitude brought many cities to the brink of ruin. At Acanthus his fleet sailed through the isthmus of Athos and after doubling the promontories of Sithonia and Pallene joined him at the city of Therma, better known by its later name of Thessalonica. Thence he continued his march through the southern part of Macedonia and Thessaly, meeting with no opposition till he reached the celebrated pass of Thermopylae.

The mighty preparations of Xerxes had been no secret in Greece; and during the preceding winter a congress of the Grecian states had been summoned by the Spartans and Athenians to meet at the isthmus of Corinth. But so great was the terror inspired by the countless hosts of Xerxes that many of the Grecian states at once tendered their submission to him, and others refused to take any part in the congress. The only people, north of the isthmus of Corinth, who remained faithful to the cause of Grecian liberty, were the Athenians and Phocians, and the inhabitants of the small Boeotian towns of Plataea and Thespiae. The other people in northern Greece were either partisans of the Persians, like the Thebans, or were unwilling to make any great sacrifices for the preservation of their independence. In Peloponnesus, the powerful city of Argos and the Achaeans stood aloof. From the more distant members of the Hellenic race no assistance was obtained. Gelon, the ruler of Syracuse, offered to send a powerful armament, provided the command of the allied forces was intrusted to him; but the envoys did not venture to accept a proposal which would have placed both Sparta and Athens under the control of a Sicilian tyrant.

The desertion of the cause of Grecian independence by so many of the Greeks did not shake the resolution of Sparta and of Athens. The Athenians, especially, set a noble example of an enlarged patriotism. They became reconciled to the AEginetans, and thus gained for the common cause the powerful navy of their rival. They readily granted to the Spartans the supreme command of the forces by sea as well as by land, although they furnished two- thirds of the vessels of the entire fleet. Their illustrious citizen Themistocles was the soul of the congress. He sought to enkindle in the other Greeks some portion of the ardour and energy which he had succeeded in breathing into the Athenians.