The failure of Datis had proved that such an expedition as could be conveyed by sea across the Egean would be insufficient to secure the object sought, and that the only safe road for a conqueror whose land force constituted his real strength was along the shores of the European continent.

But if a large army took this long and circuitous route, it must be supported by a powerful fleet; and this involved a new danger.

The losses of Mardonius off Athos had shown the perils of Egean navigation, and taught the lesson that the naval force must be at first far more than proportionate to the needs of the army, in order that it might still be sufficient notwithstanding some considerable disasters.

At the same time they had indicated one special place of danger, which might be avoided, if proper measures were taken.

Xerxes, in the four years which followed on the reduction of Egypt, continued incessantly to make the most gigantic preparations for his intended attack upon Greece, and among them included all the precautions which a wise foresight could devise in order to ward off every conceivable peril.