The History of Antiquity Volume V: Persia

The arrangement which Darius had given to his vast empire allowed the character, laws, manners, and religion of the subject nations to remain as far as possible unchanged, and only interfered, exceptionally, in the hereditary local customs of the provinces. Adequate provision for the maintenance of the central government, the establishment of rapid combinations, care for the training of the generals and officers, ample and obvious rewards for service, a system of taxation far removed from extortion, regulations for the advancement of agriculture, development of the trade on the southern sea, or by land, since the caravans could pass unharmed and even protected from Miletus to Susa, from Cyrene to the Indus, seemed to give a solid foundation, an adequate support, and abiding power to the empire of Cyrus and Darius. 

Yet for the security and continuance of it, it was of the first importance, whether the national feeling of the subject peoples, in spite of or owing to the tolerance of the empire, was still sufficiently vigorous and strong to create in them the desire to rise from the subjection in which they were, to win back their independence, and develop their national existence; whether the controlling power of the ruling people was sufficient to maintain itself for a length of time over such wide regions; whether, in fine, the ruling house would preserve, amid the splendour of its new palaces, and the brilliance of extraordinary success, the vigour and force required to sustain the heavy task of administering the empire in the manner of Darius...