Once having reasserted control over the far-flung Persian imperial domains, Darius turned to pursue the aim of expanding the reach of the empire into Europe. It seems likely that his original plan was to conquer Thrace first to secure the northern approaches to the Bosphorus, and then to proceed to establish Persia’s northern frontier at the Danube. This would be followed by a thrust westward to conquer Macedonia, thereby securing control of the entire Balkan Peninsula. Domination of the Balkans would have the immediate effect of cutting off Greece’s main supply of timber, which was essential to its survival as a maritime power. Moreover, Greece had long imported most of the wheat it needed to feed its growing population from Egypt and Libya, which were now under Persian control. Seizure of the Bosphorus and the Turkish Straits would have cut off the import of grain from its last major source of supply, the markets along the northern littoral of the Black Sea. This combination of military and economic pressures could be expected to force the Greeks into submission. The strategic conception behind this plan was bold, to say the least.
The Thracians, who were to be targeted first, were closely related to tribes that were based north of the Danube between the Carpathian Mountains and the Pruth River, the region of modern Wallachia and Moldavia. Since the Danube froze in winter, it would be relatively easy for these tribes to cross the river and carry out raids in support of the Thracians, destabilizing the frontier and necessitating the maintenance of large forces to ensure its security. Darius therefore understood that it would be necessary to extend Persian control to the opposite bank of the Danube if the security of Thrace were to be assured. Indeed, a move beyond the Danube would also serve a broader strategic purpose. A thrust toward the Syr Darya from