In all probability, however, the heaviest burden laid upon the provincial population was the obligation to furnish contingents for the army of the empire. Considering the nature of the monarchy, the actual standing army seems to have been curiously small, little more, in fact, than what was necessary for the maintenance of peace within the frontiers. In case, therefore, of external war, unsparing levies of the subject population were inevitable. The burden seems also to have been unevenly distributed: it would naturally fall most heavily on those peoples who made the most effective soldiers. From a military point of view, this feature of the imperial system was a mistake. An army formed of heterogeneous elements deficient in military training cannot under such a system have been welded into one harmonious whole. Only its multitude could be strikingly formidable. But in every other respect the new empire with which the Greeks had been brought into contact was not a mere aggregation of barbarism, but a highly organized piece of machinery, controlled by a people who, in certain sides of their civilization, compared not unfavourably with the Greeks themselves. This great scheme of organization was in all probability not carried out in one piece, though the greater part of it must be attributed to the years which followed the completion of the re-conquest. After securing the empire from disturbance from within, the next step was to secure it against disturbance from without; and it was in carrying out this policy that Darius came into conflict with the Greek in Europe. #THE GREEKS OF ASIA.# An examination of the map will show that the weakest part of the whole frontier, with the exception, perhaps, of that portion immediately east of the Caspian, lay along the shores of the Ægean and the Propontis. But whereas on the eastern frontier the races on either side of the boundary were probably alien and hostile to one another, on the western border the peoples on either side of the narrow seas were akin, and in close sympathy. The Thracian races of North-west Asia Minor were within sight of the lands of their free brethren in Europe; the Greek of the Ionian coast was within an easy and safe voyage of twenty-four hours of his mother country. Blood was thicker than water even towards the close of the sixth century before Christ; and the sympathy of kindred races so near at hand must have seemed to Darius and the Persians a standing menace to this extremity of the empire. The possibility of its taking an active form in case of a revolt was also an obvious danger. Had the Greeks of Asia been different from what they were, had, indeed, they not been Greeks, it is possible that Darius would not have considered it necessary to take action.