While we take due notice of those religious conceptions with which both the poet and the historian surround this vast conflict of Greeks and barbarians, we need look no farther than ambition and revenge for the real motives of the invasion: considering that it had been a proclaimed project in the mind of Darius for three years previous to his death, there was no probability that his son and successor would gratuitously renounce it. Shortly after the reconquest of Egypt, he began to make his preparations, the magnitude of which attested the strength of his resolve as well as the extent of his designs. The satraps and subordinate officers, throughout the whole range of his empire, received orders to furnish the amplest quota of troops and munitions of war,—horse and foot, ships of war, horse-transports, provisions, or supplies of various kinds, according to the circumstances of the territory; while rewards were held out to those who should execute the orders most efficiently. For four entire years these preparations were carried on, and as we are told that similar preparations had been going forward during the three years preceding the death of Darius, though not brought to any ultimate result, we cannot doubt that the maximum of force, which the empire could possibly be made to furnish,[21] was now brought to execute the schemes of Xerxes. The Persian empire was at this moment more extensive than ever it will appear at any subsequent period; for it comprised maritime Thrace and Macedonia as far as the borders of Thessaly, and nearly all the islands of the Ægean north of Krete and east of Eubœa, including even the Cyclades. There existed Persian forts and garrisons at Doriskus, Eion, and other places on the coast of Thrace, while Abdêra, with the other Grecian settlements on that coast were numbered among the tributaries of Susa.[22] It is necessary to bear in mind these boundaries of the empire, at the time when Xerxes mounted the throne, as compared with its reduced limits at the later time of the Peloponnesian war,—partly that we may understand the apparent chances of success to his expedition, as they presented themselves both to the Persians and to the _medizing_ Greeks,—partly that we may appreciate the after-circumstances connected with the formation of the Athenian maritime empire. [21] Herodot. vii, 19. χῶρον πάντα ἐρευνῶν τῆς ἠπείρου. [22] Herodot. vii, 106. Κατέστασαν γὰρ ἔτι πρότερον ταύτης τῆς ἐξελάσιος (_i. e._ the invasion by Xerxes) ὕπαρχοι ἐν τῇ Θρηΐκῃ καὶ τοῦ Ἑλλησπόντου πανταχῇ. vii, 108. ἐδεδούλωτο γὰρ, ὡς καὶ πρότερόν μοι δεδήλωται, ἡ μέχρι Θεσσαλίης πᾶσα, καὶ ἦν ὑπὸ βασιλῆα δασμοφόρος, Μεγαβάζου τε καταστρεψαμένου καὶ ὕστερον Μαρδονίου; also vii, 59, and Xenophon, Memorab. iii, 5, 11. Compare Æschylus Pers. 871-896, and the vision ascribed to Cyrus in reference to his successor Darius, covering with his wings both Europe and Asia (Herodot. i, 209).