The defeat at Marathon made it doubly necessary for the Persian government to undertake the subjugation of the Greeks across the sea. If there was ever to be peace on the Aegean that ‘Majuba’ must be ‘wiped off the slate.’
This time there was to be no mistake. The expedition was long and carefully prepared, and was planned on an enormous scale. The number of Xerxes' host cannot indeed be demonstrated, but it may be estimated with some probability, and the historian is bound to attempt an estimate. No sane critic could accept the millions of Herodotus. Nor would many now be found to admit the 700,000 or 800,000 given with or without garniture by Isocrates, Ctesias, and the later authors who mostly depend upon Ephorus. These figures seem to have been deduced from Herodotus. In iv. 87, the land forces led by Darius against the Scyths are said to have numbered 700,000, and it is implied that they were the full levy of the entire empire. In viii. 100 and 113, Mardonius is to be left with 300,000, while Xerxes goes home with the larger part of the army (cf. Thuc. i. 73). In vii. 20, Xerxes' host is larger than that of Darius or any other on record.