Darius, having learned what he could from these poor wretches, crossed the Bosphorus in 514, with a body of troops which tradition computed at 800,000, conquered the eastern coast of Thrace, and won his way in a series of conflicts as far as the Ister. The Ionian sailors built for him a bridge of boats, which he entrusted to their care, and he then started forward into the steppes in search of the enemy. The Scythians refused a pitched battle, but they burnt the pastures before him on every side, filled up the wells, carried off the cattle, and then slowly retreated into the interior, leaving Darius to face the vast extent of the steppes and the terrors of famine. Later tradition stated that he wandered for two months in these solitudes between the Ister and the Tanais; he had constructed on the banks of this latter river a series of earthworks, the remains of which were shown in the time of Herodotus, and had at length returned to his point of departure with merely the loss of a few sick men. The barbarians stole a march upon him, and advised the Greeks to destroy the bridge, retire within their cities, and abandon the Persians to their fate. The tyrant of the Ohersonnesus, Miltiades the Athenian, was inclined to follow their advice; but Histiasus, the governor of Miletus, opposed it, and eventually carried his point. Darius reached the southern bank without difficulty, and returned to Asia.* * Ctesias limits the campaign beyond the Danube to a fifteen days' march; and Strabo places the crossing of the Danube near the mouth of that river, at the island of Peuke, and makes the expedition stop at the Dniester. Neither the line of direction of the Persian advance nor their farthest point reached is known. The eight forts which they were said to have built, the ruins of which were shown on the banks of the Oaros as late as the time of Herodotus, were probably tumuli similar to those now met with on the Russian steppes, the origin of which is ascribed by the people to persons celebrated in their history or traditions.