But the design of Mardonius was not only directed against the Athenians and the state of Eretria, it extended also to the rest of Greece: preparations so vast were not meant to be wasted upon foes apparently insignificant, but rather to consolidate the Persian conquests on the Asiatic coasts, and to impress on the neighbouring continent of Europe adequate conceptions of the power of the great king.

By sea, Mardonius subdued the islanders of Thasus, wealthy in its gold-mines; by land he added to the Persian dependances in Thrace and Macedonia.

But losses, both by storm and battle, drove him back to Asia, and delayed for a season the deliberate and organized invasion of Greece.
In the following year (B.C.

491), while the tributary cities Mardonius had subdued were employed in constructing vessels of war and transports for cavalry, ambassadors were despatched by Darius to the various states of Greece, demanding the homage of earth and water--a preliminary calculated to ascertain who would resist, who submit to, his power--and certain to afford a pretext, in the one case for empire, in the other for invasion.

Many of the cities of the continent, and all the islands visited by the ambassadors, had the timidity to comply with the terms proposed.