The next important expedition—­one probably of still greater magnitude—­took exactly the opposite direction.  The sea which bounded the Persian dominion to the west and the north-west narrowed in two places to dimensions not much exceeding those of of the greater Asiatic rivers.  The eye which looked across the Thracian Bosphorus or the Hellespont seemed to itself to be merely contemplating the opposite bank of a pretty wide stream.  Darius, consequently being master of Asia Minor, and separated by what seemed to him so poor a barrier from fertile tracts of vast and indeed indefinite extent, such as were nowhere else to be found on the borders of his empire, naturally turned his thoughts of conquest to this quarter.  His immediate desire was, probably, to annex Thrace; but he may have already entertained wider views, and have looked to embracing in his dominions the lovely isles and coasts of Greece also, so making good the former threats of Cyrus.  The story of the voyage and escape of Democedes, related by Herodotus with such amplitude of detail, and confirmed to some extent from other sources, cannot be a mere myth without historical foundation.  Nor is it probable that the expedition was designed merely for the purpose of “indulging the exile with a short visit to his native country,” or of collecting “interesting information.”  If by the king’s orders a vessel was fitted out at Sidon to explore the coasts of Greece under the guidance of Democedes, which proceeded as far as Crotona in Magna Grsecia, we may be tolerably sure that a political object lay at the bottom of the enterprise.  It would have exactly the same aim and end as the eastern voyage of Scylax, and would be intended, like that, to pave the way for a conquest.  Darius was therefore, it would seem, already contemplating the reduction of Greece Proper, and did not require to have it suggested to him by any special provocation.  Mentally, or actually, surveying the map of the world, so far as it was known to him, he saw that in this direction only there was an attractive country readily accessible.  Elsewhere his Empire abutted on seas, sandy deserts, or at best barren steppes; here, and here only, was there a rich prize close at hand and (as it seemed) only waiting to be grasped.

But if the aggressive force of Persia was to be turned in this direction, if the stream of conquest was to be set westward along the flanks of Rhodope and Haemus, it was essential to success, and even to safety, that the line of communication with Asia should remain intact.  Now, there lay on the right flank of an army marching into Europe a vast and formidable power, known to be capable of great efforts, which, if allowed to feel itself secure from attack, might be expected at any time to step in, to break the line of communication between the east and west, and to bring the Persians who should be engaged in conquering Pseonia, Macedonia, and Greece, into imminent danger.  It is greatly to the credit of Darius that he saw this peril—­saw