So The Political Abroach to the Classical World no steps to oppose the danger. Cyrus delayed his attack until he had secured the northern empire. In a spectacular series of campaigns he reconquered the whole Median Empire and beyond it. Croesus of Lydia, whose ambition had led him to cross the Halys into Cappadocia, provoked his special attention. The result we are told by Herodotus; Sardis fell before the Persian advance, Lydia and the Ionian cities of the Aegean coast became provinces of the empire. For the next five years Cyrus was employed in conquering the east, then in 539 he turned south to Babylonia. Meeting with little resistance, he entered the great gates of Babylon and took the hands of its god. Nabonidus' general Belshazzar perished; the writing on the wall had warned in vain. With the conquest of Egypt by Cambyses, the Persian Empire was complete. From the Nile to the Black Sea and the Caspian, from the Aegean almost to the borders of India, stretched that amazing empire which Alexander still more astonishingly set himself to win. At the new capital Persepolis, Darius the Achaemenid perfected his organization of the known world. In the course of their long experience the Assyrians had made great progress in imperial administration. It was they who had first divided their territorial possessions into provinces under district governors, each with a staff of subordinate officials, and each with the duty of sending regular reports to the king. The Persian satrapies were organized after the model of these provinces, but more elaborately; each satrap was surrounded by a miniature court of officials and dignitaries. Each province had had to pay tribute to Assyria and tithes to the royal temples. Darius fixed a quota for each satrapy and an elaborate staff of officials to see that it was paid. The Assyrian system of royal roads was adopted and carried much farther by the Persians, whose empire was traversed by a network of highways, valuable for military purposes and used for swift communication by the well organized courier service.