The satrap Mazaeus set out as ordered, a man probably in his late fifties and highly distinguished for his past. For thirty years he had governed a satrapy in the west, first in Cilicia and then as Supreme Satrap of Phoenicia, Cilicia and 'Syria both beyond and inside the Euphrates'; in his new mission this local experience would serve him usefully. Possibly he had family connections at Babylon, possibly he could even speak Greek, as two-thirds of his advance party were Greek mercenaries and his coinage as satrap had long shown Greek influence; certainly, he knew the obvious bridge-point of the river Euphrates, the city of Thapsacus, probably on the river bend at Meskene and before Hephaistion could organize his two artificial bridges for Alexander's crossing, he had taken up his position on the far bank. Satrap and royal favourite thus faced each other for several days, Hephaistion not daring to finish his bridges for fear that Mazaeus would wreck them, Mazaeus waiting to witness Alexander's own approach from Tyre through the desert. When Alexander did arrive, Mazaeus turned back his horsemen and disappeared to burn the valley of the Euphrates as ordered and so to block it against Alexander's advance. He had fulfilled his mission, but there is room for a little imagination. Two thousand Greeks had waited with him on one bank of the river; on the other bank, several thousand more had fretted at the delay he was causing. As they waited, the two troops would shout to each other in their shared language. Hephaistion, perhaps, would offer promises and the two commanders may even have exchanged messages by the time that Alexander arrived. By no means the last had been heard of Mazaeus. He was to command the entire right wing in the coming battle, and no section behaved more inexplicably than the Persian right, who threw away an apparently certain victory and fled from the field which they should have encircled. Mazaeus went with them to Babylon, where a week or so later he was reinstated as governor, retaining the right to issue his silver coins. It is conceivable that the battle of Gaugamela was partly won on the banks of the Euphrates and that Mazaeus's reinstatement was less a sign of magnanimity than of a prearranged reward.

In Mazaeus's absence, Alexander finished the two bridges by lashing rafts together with chains of iron, some of which still survived on the site four hundred years later, rusted but believed to be genuine. Once across the river, he took the only unburnt route, as Darius had intended, 'keeping the Euphrates and the Armenian mountains on his left and marching northwards because everything was more convenient for his army; the horses could find fodder, supplies could be taken from the land and the heat was not so scorching'. He must have had local guides to help him, as no Greek handbook had described his route. At the foot of the Armenian mountains he swung east and followed the well-worn road to the river Tigris, halting whenever more supplies were needed. In Armenia, meanwhile, his Greek prospector had heard of the natives' rich mines of gold.

From the Euphrates to the Tigris, the distance was just under three hundred miles, paced as usual by the Macedonian surveyors, but Alexander lingered for five to seven weeks on a journey which could have been finished in a fortnight. On the road, he captured spies from Darius's army who told him 'that Darius was encamped on the river Tigris, meaning to stop him if he crossed. His army was far larger than that at Issus.' Only then did Alexander hasten forwards. When he arrived at the Tigris Darius was not to be found and the Macedonians were able to cross unopposed, their infantry wading through the headlong current, their cavalry lined on either side of them to break the rushing force of water which came above their waists. On the far bank, Alexander rested his army, leaving them to live off the fertile countryside; on 20 September, the moon eclipsed and Alexander sacrificed to the Sun, Moon and Earth, showing that he understood the natural cause of the portent. It was still possible to believe that these natural causes were themselves the effects of an omen, and the prophets and seers agreed in the prediction that during this very moon, battle would be waged and Persia's own eclipse had been foretold.

Alexander's safe and leisurely crossing of the Tigris seems almost too fortunate to be credible. At the same river seven hundred years later, the Roman Emperor Julian was opposed on the far bank by a full force of Persian cavalry, when the country behind him had been so thoroughly burnt that his army could only retreat at the risk of starving, while the river current before him was flowing so arrow-swift that he had to destroy his many ships, knowing they could never negotiate it. Alexander had crossed at the obvious ford, the modern Abu Dhahir, where the Persians' own Royal Road ran through the river and led south-east to Gaugamela and Arbela, Darius's eventual place of battle. His movements had been predictable from start to finish, for only two routes led to the Tigris and one had already been burnt; spies, even, had warned him of opposition on the Tigris and yet, after a month in which to manoeuvre, Darius had left him with food and a free passage.

Timing alone cannot excuse the Great King. Perhaps he had unwisely delayed at headquarters until Mazaeus had galloped back from the Euplirates, 600 miles distant, with the news that Alexander had indeed turned northwards as expected; perhaps his headquarters were still as far south as Babylon. But even then, in the latter half of August, three weeks or more remained before Alexander would have reached the Tigris, time enough for a large force of cavalry to hurry 400 miles, at most, and occupy the Royal Road's crossing, Alexander's obvious choice of the four possible fords. Horsemen, as Julian was to find, could hold the river with the help of its current, and serious devastation would starve any enemy into retreat. The plan, it seems, had been discussed, for when Darius's scouts had been captured, they warned Alexander of plans for resistance on the Tigris; like true spies, they may only have invented the plan to deceive their captor, but three days later, on the far bank, a small force of cavalry did appear and try to bum the grain stacks. Perhaps Darius had hesitated, then ordered them north to the river too late; quickly applied, the idea could have worked, especially if linked to that burning of crops and scorching of earth which King Shapur II was to use so effectively against Julian's Romans in the same area. Here, at least, there was no excuse; Darius, it seems, had been stupid as well as slow.

Stupidity, however, is easier to detect when the sequel is known. At the time, Darius's apparent folly was a measure of his mood, that imponderable element which history has tended to forget. To a King of Kings, chosen by Ahura Mazda and supported by an army which the victors later assessed at a million strong, Alexander's 47,000 troops were asking to be crushed. At Issus, the ground and the gods had been unfavourable but at the chosen site of Gaugamela, seventy-six miles south cast of the Tigris crossing, the battlefield was being smoothed for the Empire's traditional weapons, the cavalry and the scythed chariots which its feudal army system existed to maintain. Hyrcanians from the Caspian, Indians from the Punjab, Mcdes from the empire's centre and allied Scythians from beyond the Oxus; such tribal cavalry from the Upper Satrapies' villages made up for the diminution of the hired Greek infantry, by losses and desertion, to a mere 4,000 loyalists. When these and many others were waiting, at least five times as numerous as the enemy, what need of devastation or unsettling division of the troops? They might only deter Alexander from advancing into a foe far larger than he knew. But if Alexander had underestimated the numbers of the Persians, Darius had underestimated his opponent's military genius.