The motives of the expedition seem to have been twofold—
(1) A declared motive,—the punishment of Athens and Eretria.
(2) A motive which the Persian had good reason for keeping, in so far as possible, in the background,—the establishment of a tête-du-pont on the European shore in Attica.
The first of these is expressly stated by Herodotus. Of the existence of the second he appears to have had an inkling.
H. vi. 94.
He says that the expedition was due to Darius’ desire for revenge, and to the charges and importunities of the Peisistratidæ. He further says that Darius wished “on this pretext” (i.e. the representations made by the Peisistratidæ) “to subdue those people in Greece which had not given earth and water.” From an historical point of view, exception must be taken to one phase of this last statement; in fact, in other words in the same chapter Herodotus narrows it down to a noticeable extent. He omits all mention of the recalcitrant states, and says that Darius appointed Datis and Artaphernes to the command “with orders to enslave Athens and Eretria, and to bring the slaves into his presence.”
The facts with regard to the expedition make it evident that Darius was anxious to limit, in so far as possible, the declared scope of it. It did not aim at a large measure of conquest in Greece. The contrast in point of numbers between it and the expedition of Mardonios has been already referred to. The Persian war policy of this age is characterized by one invariable feature—a large numerical margin relative to the end to be attained. Darius’ recent experiences in Ionia were not likely to make him grossly underestimate the forces requisite for any large design of conquest in Greece.
If this be recognized, certain features of the Persian policy at this particular time become explicable. Let it be assumed that H. vi. 49. Herodotus’ statement as to the wide nature of the demand for “earth and water” is correct.[59]
It was made before the expedition started; and presumably its results were known before the fleet sailed from the Asian coast. OBJECTIVE OF EXPEDITION OF 490. Thus the Persian commanders knew what they had to expect; and, if Herodotus’ statements be correct, a large number of states had been detached from the possible defence.
Then there was Hippias. His presence with the expedition served a double purpose. He was an outward and visible sign of the limitations of the Persian intentions. He was to be re-established as tyrant in Athens. But the Persian might expect more from him. His presence might detach Sparta from the defence. Was it not Sparta which had, not so many years before, called the Peloponnesian states together and proposed the forcible reinstatement of Hippias?