Before this chapter is concluded a few words must be said with respect to the countries which bordered upon the Persian Empire.  The Empire was surrounded, for the most part, either by seas or deserts.  The Mediterranean, the Egean, the Propontis, the Euxine, the Caspian, the Indian Ocean, the Persian Gulf, and the Arabian Gulf or Bed Sea washed its shores, bounding almost all its western, and much of its northern and southern sides; while the sands of the Sahara, the deserts of Arabia and Syria of India and Thibet, filled up the greater part of the intervening spaces.  The only countries of importance which can be viewed as in any sense neighbors of Persia are European and Asiatic Scythia, Hindustan, Arabia, Ethiopia, and Greece.

Where the Black Sea, curving round to the north, ceased to furnish to the Empire the advantage of a water barrier, a protection of almost equal strength was afforded to it by the mountain-chain of the Caucasus.  Excepting on the extreme east, where it slopes gently to the Caspian, this range is one of great elevation, possessing but few passes, and very difficult to traverse.  Its fastnesses have always been inhabited by wild tribes, jealous of their freedom; and these tribes may have caused annoyance, but they could at no time have been a serious danger to the Empire.  They were weak in numbers, divided in nationality and in interests, and quite incapable of conducting any distant expedition.  Like their modern successors, the Circassians, Abassians, and Lesghians, their one and only desire was to maintain themselves in possession of their beloved mountains; and this desire would cause them to resist all attempts that might be made to traverse their country, whether proceeding from the north or from the south, from the inhabitants of Europe or from those of Asia.  Persia was thus strongly protected in this quarter; but still she could not feel herself altogether safe.  Once at least within historic memory the barrier of the Caucasus had proved to be surmountable.  From the vast Steppe which stretches northwards from its base, in part salt, in part grassy, had crossed into Asia—­through its passes or round its eastern flank—­a countless host, which had swept all before it, and brought ruin upon flourishing empires.  The Scythian and Samaritan hordes of the steppe-country between the Wolga and the Dnieper were to the monarchies of Western Asia a permanent, if a somewhat distant, peril.  It could not be forgotten that they had proved themselves capable of penetrating the rocky barrier which would otherwise have seemed so sure a protection, or that when they swarmed across it in the seventh century before our era, their strength was at first irresistible.  The Persians knew, what the great nations of the earth afterwards forgot, that along the northern horizon there lay a black cloud, which might at any time burst, carrying desolation to their homes and bringing ruin upon their civilization.  We shall find the course of their history importantly affected by a sense of this danger, and we shall have reason to admire the wisdom of their measures of precaution against it.