Once over the Euphrates, Alexander had the option of marching down the left bank of that river to Babylon, the chief city of the Persian empire, and the natural place to find Darius.[359] But this march (as we know from Xenophon, who made it with the Ten Thousand Greeks) would be one of extreme suffering and through a desert country where no provisions were to be got. Moreover, Mazæus in retreating had taken a north-easterly direction towards the upper part of the Tigris; and some prisoners reported that Darius with his main army was behind the Tigris, intending to defend the passage of that river against Alexander. The Tigris appears not to be fordable below Nineveh (Mosul). Accordingly he directed his march, first nearly northward, having the Euphrates on his left hand; next eastward across Northern Mesopotamia, having the Armenian mountains on his left hand. On reaching the ford of the Tigris, he found it absolutely undefended. Not a single enemy being in sight, he forded the river as soon as possible, with all his infantry, cavalry, and baggage. The difficulties and perils of crossing were extreme, from the depth of the water, above their breasts, the rapidity of the current, and the slippery footing.[360] A resolute and vigilant enemy might have rendered the passage almost impossible. But the good fortune of Alexander was not less conspicuous in what his enemies left undone, than in what they actually did.[361] [359] So Alexander considers Babylon (Arrian, ii. 17, 3-10)—προχωρησάντων ξὺν τῇ δυνάμει ἐπὶ Βαβυλῶνά τε καὶ Δαρεῖον ... τόν τε ἐπὶ Βαβυλῶνος στόλον ποιησόμεθα, etc. This is the explanation of Arrian’s remark, iii. 7, 6—where he assigns the reason why Alexander, after passing the Euphrates at Thapsakus, did not take the straight road towards Babylon. Cyrus the younger marched directly to Babylon to attack Artaxerxes. Susa, Ekbatana, and Persepolis were more distant, and less exposed to an enemy from the west. [360] Arrian, iii. 7, 8; Diodor. xvii. 55; Curtius. iv. 9, 17-24. “Magna munimenta regni Tigris atque Euphrates erant”, is a part of the speech put into the mouth of Darius before the battle of Arbela, by Curtius, (iv. 14, 10). Both these great defences were abandoned. [361] Curtius, iv. 9, 23; Plutarch, Alexand. 39.