If Cambyses had supported his expedition against Egypt by the navy of the Phenician cities and the Greek cities of Anatolia, Darius had still more urgent need of their sailors to convey him to Europe, across the Danube. To the mariners of the Anatolian coasts and the islands lying off them, the waters of the Black Sea and the mouths of the Danube and Borysthenes were hardly less familiar than the shores of the Ægean. This co-operation was therefore the most essential. Darius called out the navy of the Greek cities of his kingdom, and that navy only; employment was found for the Phenician fleet was another direction. The Greeks had to furnish no less than 600 triremes, _i.e._ a fleet of which the crews reached the total of 120,000 men. That fleet was intended to convey the land army, the levy of the entire kingdom, across the straits, and it must assemble before the army arrived. The task before it was the transport of 700,000 men, for that, according to Herodotus, was the strength of the army of Darius, with numerous horses, and the enormous train of servants, porters, and beasts of burden to Europe. This involved the embarkation and debarkation of the animals,--a long and difficult operation; it was desirable to lose as little time as possible, and more desirable still to keep and maintain a safe connection with Asia in the rear of the army. Hence Darius considered whether it were possible to bridge over one of the straits. He found a Greek who undertook to carry out this idea, and had no scruples in building a bridge to connect the mighty Persian empire with Europe, and facilitate the subjugation of his own countrymen in their native land. In the island of Samos, so recently conquered by Darius, were the best engineers in Greece. After the construction of the great temple of Hera had been begun, the Samians had found various opportunities of exercising their skill. A long and difficult acqueduct, and breakwaters for the protection of the harbour, had been partly begun and partly carried out before the reign of Polycrates; the building of the palace, the strong fortifications, and, above all, the great docks and harbour-works, which Polycrates set on foot, had given yet further practice to the Samians. From this school came Mandrocles, who undertook the construction of the bridge.