In February of 401 B.C., Xenophon, the Athenian, set out in a contingent often thousand Greek mercenaries in the army of Cyrus, the Pretender to the throne of Persia. Cyrus, in fact, was leading the army against his brother Artaxerxes, the Great King of Persia, though the real object of the expedition was not revealed officially until the army reached Thapsacus on the Euphrates in July and crossed the river from Syria into Mesopotamia. Xenophon joined the expedition more or less as a diplomatic attaché, not a soldier. But he became an observer and critic, and, since he was a man of talents, he was driven by self-preservation and by disasters which befell them in the end, virtually to take command. His account of these events was not published or written till many years had elapsed: but it is clear that it must have been based on a regularly kept log or diary.The geographical problems which Xenophon's report of their itinerary raises have occupied scholars and travellers for nearly two hundred years, the earliest attempt to identify the sites mentioned by him being that of d'Anville in 1779. For this long history of an unsettled problem, the reasons are that, firstly, we still know little from cuneiform sources about the ancient geography of Central Mesopotamia, and as yet no monograph exists which studies Babylonia in this period; secondly, that the record of Xenophon, though invaluable, is bedevilled occasionally by false reports or inadequate or misunderstood data, or possibly by errors in transmission of the text; and in our own time, progress has been held up by insufficient study of the ground, in particular of the ancient courses of the Euphrates and Tigris and the canals that fed them—and finally, by the inadequate use of aerial photography.