Passage of the Euphrates and Tigris. Alexander arrived at Thapsacus in the month Hecatombaion,381 in the archonship of Aristophanes at Athens; and he found that two bridges of boats had been constructed over the stream. But Mazaeus, to whom Darius had committed the duty of guarding the river, with about 3,000 cavalry, 2,000 of which were Grecian mercenaries, was up to that time keeping guard there at the river. For this reason the Macedonians had not constructed the complete bridge as far as the opposite bank, being afraid that Mazaeus might make an assault upon the bridge where it ended. But when he heard that Alexander was approaching, he went off in flight with all his army. 153As soon as he had fled, the bridges were completed as far as the further bank, and Alexander crossed upon them with his army.382 Thence he marched up into the interior through the land called Mesopotamia, having the river Euphrates and the mountains of Armenia on his left. When he started from the Euphrates he did not march to Babylon by the direct road; because by going the other route be found all things easier for the march of his army, and it was also easier to obtain fodder for the horses and provisions for the men from the country. Besides this, the heat was not so scorching on the indirect route. Some of the men from Darius’s army, who were dispersed for the purpose of scouting, were taken prisoners; and they reported that Darius was encamped near the river Tigris, having resolved to............