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 it and took effectual measures to guard against it.

The Scythian expedition was no insane project of a frantic despot, burning for revenge, or ambitious of an impossible conquest.