East of modern Kabul, the army returned to the valleys along the modern main road into India and relaxed in smoother ground. They were still ringed round by mountains, but with the worst of the journey before them, they were allowed a three-month winter camp at the Persians' satrapal capital of Kapisa. Again, Alexander was moved to refound a Persian fort, settling it as a Greek city with 7,000 natives and with veterans and such hired troops as he could spare from his dwindling army. The result, known as Alexandria-in-the-Caucasus, has never been found on the ground, though brief French excavations at Borj-i-Abdulla south of modern Begram have uncovered traces of Greek towers and the wall of a city which succeeded Alexandria 150 years later. On the gentle slopes of these foothills, the most abundant basin in the area, the city was surely intended to guard the ancient routes through the Hindu Kush, for in the valleys of Bcgram and Kabul no less than three main roads converge, while the citadel overlooked the confluence of two main rivers, the Persian's favourite site for such a fort. For the past two hundred years since Cyrus, the Persians had maintained a garrison town on the same strategic site; Alexander, as so often in the far east, had taken the cue for an Alexandria from his Persian predecessors. The new town's style of life, if not its site, was entirely different: there had never been a theatre in a Persian outpost, but 150 years after Alexander, Alexandria-in-the-Caucasus still contained carvings of Greek comic actors, dressed for the stage which had flourished inside its walls. Even by a road-post for veterans the banner of Greek culture was defiantly raised in an Afghan valley.

Not until May, a seven-month delay, would Alexander have 'sacrificed to the usual gods', the most easterly sacrifice that had yet been wafted to the Olympians, and exhorted his army to the climax of their march. Like Tamurlane after him, he was to climb the northern buttress of the Hindu Kush by the Khaiwak pass, which rises to a height of 11,000 feet before dropping down to the plains of Bactria and further Asia beyond. The south and nearer face, which rears to the sky beyond Begram, was not attempted until the snows began to melt. Hunger, not cold, was the problem. The march to the summit took a week, and supplies throughout were desperately scarce except for the plants of terebinth and asa foetida, savoury but insubstantial herbs. Though the histories hurry over the experience a background of landscape can still be restored. There is an intensity of sunrise and sunset in these Afghan mountains which even the hill-farmers of upper Macedonia cannot have watched unmoved. The light throws patches of blue and violet on to the melting snow, striking the pink mountain rock and the grey clumps of prickly thrift, a plant as sharp as a hedgehog which carpets the slopes and discomforts the unwary traveller. 'A thin purple veil', a German explorer has written, travelling, like Alexander, through the Hindu Kush in the early season, 'very subdued and as misty as a breeze was daily drawn across the eastern sky. As I gazed, the clouds turned to flame and blood-red; the snowy peaks were glowing while a deep and inexpressible yearning filled me through and through.' That same sense of yearning was seen in Alexander by his fairest Roman historian, perhaps correctly; it was an emotion which drove him in search of myth or places of mystery; it was proper to a king living out the Homeric past. In the Hindu Kush myth and landscape combined as if to bring it into play.

'In the middle of the mountains, there was a rock half a mile high in which the cave of Prometheus was pointed out by the natives, along with the nest of the mythical eagle and the marks of his chains.' In Greek legend, the hero Prometheus was punished for his inventive intelligence and imprisoned by Zeus on an eastern rock where an eagle would gnaw at his liver; here in the Hindu Kush, the site of his punishment seemed at last to have been discovered. But this greatest of all myths had always been placed in the Caucasus, miles away to the north-west; in order to reconcile myth and geography, Alexander's staff maintained that the Hindu Kush was attached to the Caucasus as an easterly continuation. Ever since, scholars ancient and modern have treated their mistake unkindly, dismissing it as a flattery which deliberately brought Alexander into contact with the distant Caucasus mountains, an area which he never reached. But that is to misunderstand a very human error. The Greek name for the Hindu Kush, the Paropamisus, was derived from the Persian word 'uparisena', meaning 'peak over which the eagle cannot fly'. The eagle was part of local knowledge, and as with Prometheus it was a symbol with a history of its own; in local Iranian myth, the eagle Sena had saved the hero Dastan, son of Sam, from cruel imprisonment. Alexander's staff were evidently told the story by the natives and at once equated the details with their own Prometheus; if these mountains confined the mythical eagle, they must be the Caucasus whatever the geography, for that was where Prometheus and his eagle were known to lie. The marks of the chains were easily detected in the jaggedness of a Hindu Kush rockface: to Alexander's officers, geography was only accurate if first it fitted myth, and although the Hindu Kush is grander than Greek mountains and as weathered a brown as its natives' bread, it is a landscape with a definite feel of Greece.

While the foothills of the south face offered myth, the summit of the so-called Caucasus promised even greater rewards. Aristotle, who knew nothing of China and Far Asia, had believed that the eastern edge of the world could be seen from the top of the Hindu Kush, and perhaps his former pupil remembered this, if anything, from any school hours spent in geography; the belief may have been common too among ordinary Greeks, and for a man aged twenty-six there could be no more momentous ambition. A short march eastwards, and he could survey the boundary of the world and see how the border lands of India merged with the eddying ocean. To an explorer such an ambition needs no further justification, while to an Achilles any fighting to attain it was so much more service in the cause of glory. From the top of the Khaiwak pass, Aristotle's geography would already seem suspect to anyone in the army who remembered it. Ridge upon ridge of mountains rose eastwards, but for the moment Bessus mattered more than the problem of the world's end, and the army was coaxed northwards in search of him down the far face of the Hindu Kush.

The descent lasted at most ten days; it was intolerable throughout. The snow, facing northwards, still lay heavily and masked the line of the pass; only a parallel can show what this might mean. In April 1398 Tamurlane crossed the same face of the Khaiwak, forcing himself and his Mongols to crawl its glaciers on hands and knees, drag their pack-animals by wooden sledge and swing across its open ravines on rope bridges lassoed round prominent rocks; more men were lost in the crossing than in the whole of the campaign year. Alexander's horses would have been fitted with the leather snowboots, which Greek generals found useful against deep or slippery drifts; nonetheless, they suffered severely, as their needs took second place to their riders'. The natives had stored their supplies in underground pits which were hard to find and harder to break open: famine, therefore, spread through the army, as the few available jars of wine and honey were sold at absurdly high prices. On the lower slopes, where herbs and the famous brown trout of the rivers filled out the soldiers' diet, the animals found no fodder and orders were soon given to slaughter them and use them as meat. The scrub bushes of prickly thrift, which served the natives as firewood, were still buried under the snow; in the absence of any other fuel, horse and pack-ass were seasoned with the juice of silphium and eaten raw.