The empire of Darius I was larger than any one of the preceding empires whose growth we have traced. It included all Asia Minor and Syria, that is to say, the ancient Lydian and Hittite empires, all the old Assyrian and Babylonian empires, Egypt, the Caucasus and Caspian regions, Media, Persia, and it extended, perhaps, into India to the Indus. The nomadic Arabians alone of all the peoples of what is nowadays called the Near East, did not pay tribute to the satraps (provincial governors) of Darius. The organization of this great empire seems to have been on a much higher level of efficiency than any of its precursors. Great arterial roads joined province to province, and there was a system of royal posts;[179] at stated intervals post horses stood always ready to carry the government messenger, or the traveller if he had a government permit, on to the next stage of his journey. Apart from this imperial right-of-way and the payment of tribute, the local governments possessed a very considerable amount of local freedom. They were restrained from internecine conflict, which was all to their own good. And at first the Greek cities of the mainland of Asia paid the tribute and shared in this Persian Peace. Darius was first incited to attack the Greeks in Europe by a homesick Greek physician at his court, who wanted at any cost to be back in Greece. Darius had already made plans for an expedition into Europe, aiming not at Greece, but to the northward of Greece, across the Bosphorus and Danube. He wanted to strike at South Russia, which he believed to be the home country of the Scythian nomads who threatened him on his northern and north-eastern frontiers. But he lent an attentive ear to the tempter, and sent agents into Greece.[180] This great expedition of Darius opens out our view in this history. It lifts a curtain upon the Balkan country behind Greece about which we have said nothing hitherto; it carries us to and over the Danube. The nucleus of his army marched from Susa, gathering up contingents as they made their way to the Bosphorus. Here Greek allies (Ionian Greeks from Asia) had made a bridge of boats, and the army crossed over while the Greek allies sailed on in their ships to the Danube, and, two days’ sail up from its mouth, landed to make another floating bridge. Meanwhile, Darius and his host advanced along the coast of what is now Bulgaria, but which was then called Thrace. They crossed the Danube, and prepared to give battle to the Scythian army and take the cities of the Scythians.