But soon the restless spirit which possessed all the earlier Persian monarchs incited Darius to carry his warlike enterprises into "fresh fields and pastures new." From the eastern coast of the AEgean Sea he looked out towards a land possessing every attraction that soil or clime could offer, fertile, rich in minerals, and with many excellent harbours, well watered, abounding in corn and wine and oil, in wooded hillsides, and in productive plains.

According to Herodotus,[14290] he had already explored the strength and weakness of the region by means of a commission of Persian nobles, who had surveyed all the shores of Greece from the decks of Phoenician ships.
The result was that he coveted the possession of the land thus made known to him, and came to a fixed resolution that he would add it to his territories.
There were two modes by which Greece might be approached from Asia.
Bridges of boats could be thrown across the Bosphorus or the Hellespont, mere salt rivers, scarcely more formidable than the streams of the Euphrates and the Tigris.

In this way Europe could be invaded in force, and the army sent across the straits, could pursue its way along the shore till it reached the rich plains of Thessaly, and from Thessaly passed into Boetia, Attica, and the Peloponnese.

Or a fleet, with a land force on board, might proceed from Asia Minor across the AEgean, where the numerous islands, scattered at short intervals, seemed to have been arranged by nature as stepping-stones, whereby the adventurous denizens of either continent might cross easily into the other; and a landing might be suddenly effected near the very heart of Greece without a tenth part of the trouble that must be taken if the other line of route were pursued.

In either case the attendance of a fleet would be necessary.