With Greece pacified, Alexander could proceed to the invasion of Persia. Since the days of Darius the Great the empire had remained almost intact-- a huge, loosely-knit collection of many different peoples, whose sole bond of union was their common allegiance to the Great King. [5] Its resources were enormous. There were millions of men for the armies and untold wealth in the royal treasuries. Yet the empire was a hollow shell. EXPEDITION OF THE "TEN THOUSAND," 401-400 B.C. Some seventy years before Alexander set forth on his expedition the Greeks had witnessed a remarkable disclosure of the military weakness of Persia. One of those rare revolts which troubled the security of the Persian Empire broke out in Asia Minor. It was headed by Cyrus the Younger, a brother of the Persian monarch. Cyrus gathered a large body of native troops and also hired about ten thousand Greek soldiers. He led this mixed force into the heart of the Persian dominions, only to fall in battle at Cunaxa, near Babylon. The Greeks easily routed the enemy arrayed against them, but the death of Cyrus made their victory fruitless. In spite of their desperate situation the Greeks refused to surrender and started to return homewards. The Persians dogged their footsteps, yet never ventured on a pitched battle. After months of wandering in Assyria and Armenia the little band of intrepid soldiers finally reached Trapezus, (Modern Trebizond) a Greek city on the Black Sea. [Illustration: Map, ROUTE OF THE TEN THOUSAND] SIGNIFICANCE OF THE EXPEDITION The story of this invasion of Persia and the subsequent retreat was written by the Athenian Xenophon [6] in his _Anabasis_. It is one of the most interesting books that have come down to us from antiquity. We can judge from it how vivid was the impression which the adventures of the "Ten Thousand" made on the Greeks of Xenophon's time. A small army had marched to the center of the Persian dominions, had overcome a host many times its size, and had returned to Greece in safety. It was clear proof that the Persian power, however imposing on the outside, could offer no effective resistance to an attack by a strong force of disciplined Greek soldiers. Henceforth the Greeks never abandoned the idea of an invasion of Persia. ALEXANDER'S INVASION The gigantic task fell, however, to Alexander, as the champion of Hellas against the "barbarians." With an army of less than forty thousand men Alexander destroyed an empire before which, for two centuries, all Asia had been wont to tremble. History, ancient or modern, contains no other record of conquests so widespread, so thorough, so amazingly rapid. 42. CONQUEST OF PERSIA AND THE FAR EAST, 334-323 B.C. BATTLE OF THE GRANICUS, 334 B.C.