On the right, at the foot of the mountains, Alexander's meeting with Darius's cavalry was bold, and entirely successful. The enemy archers, light infantry and heavy cavalry gave way at the first shock; there was much jostling, whereupon the Companions, pulling hard on their bits, managed to swing their horses round to the left and strike into the Persian centre where, according to royal custom, Darius had stationed his chariot. Their virtuosity was well-timed; back in the Macedonian centre, the phalanx was faltering on the brink of the river and beginning to come adrift as it tried to match the speed of the king and cavalry: ranks were breaking, the wall of sarissas was parting and Darius's hired Greeks had hurled themselves across the river into the gaps, 'challenging the phalanx's widespread reputation for invincibility'. Fighting was fierce and the Macedonian losses would have been severer had not Alexander's horsemen, wheeling into the Persian centre, cut off the hired Greeks from behind and forced them to look back to their encircled rear.

On the seashore to the left Parmenion's brigades had held firm in the face of oriental slingers and heavy cavalry. Far from piercing a gap beside the sea, Darius's horsemen found themselves thrown back to join their centre, while the Thessalians ripped past them down the left wing, meaning to circle round and join with Alexander in the pursuit. As the winter darkness came on and cavalry fought their way towards him on both sides, the Great King realized his danger and decided to turn his chariot and flee, leaving his brother Oxathres to fend heroically for himself among the advancing Macedonian horsemen. For one vivid moment, caught in an ancient mosaic of the battle, probably based on an original painting by a contemporary, king met the eye of king, Darius urging his tossing horses to swing about. Alexander pushing through the press, intent on spearing his rival to death. This contest, at least, Darius was to win; his brother Oxathres and other Persian nobles came to brave terms with the Macedonians, and behind the protection of his Royal Relations Darius was free to rattle over the hilly ground in his chariot until the streams and gullies impeded his advance and he was forced to take to his horse. His shield and his Persian robe he abandoned in his empty chariot for Alexander to find behind him; as he made the most of his start, night's onset stayed the Macedonians and Thessalians from more pressing pursuit.

In the battle 110,000 Persians were killed, said the historians, whereas the Macedonian dead totalled 302; their general agreement suggests that these farcical figures were derived from Callisthenes, writing up the triumph for his king's enjoyment. Ptolemy, who shared in the pursuit, even overstepped Callisthenes's limit and claimed that he had ridden across a ravine on the bodies of the Persian dead which filled it. Despite the official figures, the Macedonian infantry, split and unprotected, would surely have suffered heavily from the opposing Greeks and it is perhaps relevant that among these glamorous fibs a figure of 4,000 Macedonian wounded is also recorded, possibly nearer the painful truth. For the battle of Issus had exposed the recurrent limitations of the Foot Companions forced to march on rough terrain; victory, as never before in Greek warfare and seldom afterwards even in modern times, had been won wholly by the merits of the cavalry, outnumbered and seriously hampered by the lie of the land, yet still able to meet the Persians' right, swing left-handed and pierce the flanks of its centre. Such horsemanship would not be seen again until the Carthaginians' double charge at Cannae, when the ground was level and their Roman opponents were neither so skilled nor so heavily armoured as Darius's Orientals. Alexander's victory cannot be attributed to any notable superiority of weapons, although some if not all, of the Persian horses and riders were heavily armoured, so much so that their weight slowed their final retreat. They were beaten because they were bowled over by a charge, and then jostled off balance at close quarters; this bowling over was the result of that training, dash and high morale which make the Companions the finest cavalry in history, and for this, their commander Alexander must be held directly responsible.

From the battlefield the Persian forces scattered to all four points of the compass, many following Darius eastwards to the safe heart of the empire, many risking the northerly route through Cilicia into the fastnesses of the Taurus mountains, others heading westwards for the Asia Minor coast and others, some 4,000 soldiers of fortune, rallying to Amyntas the Macedonian deserter and circling southwards to try their hand in that richest of Asian prizes, the satrapy of Egypt. For some twenty miles those with Darius were dogged by Alexander and his Companions, hoping for the prey which would tum their victory into a triumph. But with half a mile's start through unfamiliar country, the Great King had time to escape eastwards through the Amanid mountains, and eventually Alexander gave up the chase, arriving back in his camp on the verge of midnight. His failure was a grave disappointment but back on the field of victory there were prizes enough to make up for the loss of Darius's person.

Even in his army camp, Darius had encumbered himself with riches and paraphernalia, though these were only a foretaste of what lay abandoned at his base in Damascus. The Macedonians had plundered all that was to hand, reserving the royal tent for the man who now deserved it, so that when Alexander returned at midnight, bloodstained and muddied, expressing a wish to wash off his sweat in Darius's bath, they could lead him forwards to his rightful prize, a Companion reminding him that Darius's bath was in future to be known as Alexander's. On the threshold of the royal tent, Alexander stood surprised, struck by a sight which no young man from Pella could ever have imagined to be true:

When he saw the bowls, pitchers, tubs and caskets, all of gold, most exquisitely worked and set in a chamber which breathed a marvellous scent of incense and spices, when he passed through into a tent whose size and height were no less remarkable, whose sofas and tables were even laid for his dinner, then he looked long and hard at his Companions and remarked: 'This, it would seem, is to be a King.'

But there is more to kingship than its treasures. Alexander was tired; he wanted his bath and dinner; he was limping from a dagger-wound in his thigh which court gossip attributed to a thrust from King Darius himself. And yet he was perturbed by the sound of ladies wailing close to where he stood, and on asking what ladies could possibly be responsible he was told that these were Darius's wife, mother and children, weeping for the king whom they believed to be dead. Promptly, Alexander sent a Companion, Leonnatus, to reassure them and to tell them, perhaps in Persian, that Darius lived, though his cloak and weapons had been captured in his chariot: Alexander would grant them royal state and the continuing rank of Queen, as it was Darius, not his family, upon whom he was making war.

The following morning, Alexander summoned Hephaistion and went to visit his royal captives. When they entered her tent, it was said, the Queen Mother did obeisance to Hephaistion, mistaking him for Alexander as be seemed so plainly the taller of the two. Hephaistion recoiled and an attendant corrected her; she stood back, flustered at her mistake. Alexander, as with his Carian mother Ada, had the tact to cope with a lady's embarrassment: 'No mistake,' he replied, 'for he too is an Alexander.' Then, he complimented Darius's wife on her six-year-old son and confirmed the ladies' privileges, presenting them with dresses and jewellery, and giving them leave to bury any of their Persian dead; they were to live unmolested in quarters of their own, honour being paid to their beauty. Once more, Alexander had shown himself able to respect feminine nobility; his captives could have been valuable hostages, but he never used them for political bargaining, and not for nine years did he marry Darius's daughter.