Babylon was already waiting to become his capital, yet in June 323 BC Alexander lay fevered in the palace of Nebuchadnezzar II, unable to speak, while his soldiers filed past in silence. The man who had crossed from Greece to north-western India was only thirty-two, and the empire he had built was already larger than any in the world. In that last week the old question hovered over him, not whether he had conquered enough, but who could hold it together when he was gone.

Wind back to Pella in Macedon, where Alexander III was born on 20 or 21 July 356 BC, the son of Philip II and Olympias. Philip had seven or eight wives, yet Olympias was the principal one for a time, and the boy grew up amid court ambition and hard lessons. Lanike, Leonidas, and Lysimachus taught him, while Aristotle took over at Mieza when he was thirteen. There, among the sons of nobles such as Ptolemy, Hephaestion, and Cassander, he learnt medicine, logic, morals, and Homer, and carried an annotated Iliad on campaign for the rest of his life.

The tale of Bucephalas shows the boy he was becoming. At ten, when a Thessalian trader offered the horse for thirteen talents and Philip turned it away, Alexander noticed that the animal feared its own shadow. He tamed it, and Philip, delighted, told him Macedon was too small for his ambitions. That horse carried him as far as India, and when it died at thirty, Alexander founded Bucephala in its memory. Even before he was king, he was already learning to turn fear into possession.

At sixteen, while Philip campaigned in Thrace, Alexander acted as regent and crushed the revolt of the Maedi, founding Alexandropolis. In 338 BC he fought at Chaeronea beside his father, commanding the left wing while Philip used retreat and cunning on the right. After the victory Philip created the League of Corinth, named himself hegemon, and prepared war against Persia. But the family turned sharp when Philip married Cleopatra Eurydice in 338 BC, and Alexander fled Macedon in 337 BC after a quarrel at the wedding banquet. He returned six months later through Demaratus, yet the bond with his father had been bruised for good.

Then, in October 336 BC at Aigai, Philip was murdered by Pausanias, one of his bodyguards, and Alexander was proclaimed king at twenty. He moved at once to remove rivals, executing his cousin Amyntas IV, killing two Lyncestian princes, and ordering Attalus dead. When Thebes, Athens, Thessaly, and the Thracians rose in revolt, he answered with speed rather than hesitation. He rode into Thessaly with 3,000 cavalry, crossed Mount Ossa, and made the enemy surrender on waking. At Corinth he met Diogenes the Cynic, who asked him to stand aside because he was blocking the sun, and Alexander admired the reply.

The first great warning came in 335 BC. While Alexander was securing Thrace, the Triballi, the Getae, and the Illyrians all tested him, and he beat them back from Mount Haemus to the Danube and west into Illyria. Then Thebes rebelled again. Alexander returned south and razed the city, dividing its land among the Boeotian towns. It was a brutal lesson to Greece, and it worked: Athens quietened, the League of Corinth stayed obedient, and Antipater was left behind as regent while Alexander crossed into Asia in 334 BC.

He crossed the Hellespont with about 48,100 soldiers and 6,100 cavalry, threw a spear into Asian soil, and accepted Asia as a gift from the gods. At the Granicus he beat a much larger Persian force, then took Sardis, granted autonomy to the Ionian cities, and besieged Miletus and Halicarnassus. From there he drove through Lycia, Pamphylia, and Gordium, where he cut the Gordian Knot with his sword. In spring 333 BC he entered Cilicia, fought Darius III at Issus, and won again. Darius fled, leaving his wife, his mother Sisygambis, his daughters, and 10,000 talents in ransom.

The campaign tightened like a noose. In 332 BC Alexander besieged Tyre for months and then Gaza, where he was wounded in the shoulder. Egypt welcomed him as a liberator, and at Memphis he was crowned in the temple of Ptah after visiting the oracle of Amun at Siwa, which hailed him as the son of the god Amun. He founded Alexandria, restored temples neglected by the Persians, and organised Egypt before marching east again in early 331 BC. There, in the dust of Babylon and Susa, he was no longer merely a conqueror. He was trying to become the rightful heir of an empire he had broken.

At Gaugamela in 331 BC, Alexander won the decisive battle against Darius III, whose chariots with scythes could not stop the Macedonian advance. Babylon opened its gates, then Susa, then Persepolis, where the treasury fell and the palace of Xerxes I burned, perhaps by accident, perhaps in revenge for Athens. Alexander later regretted the fire, but the Achaemenid throne was finished. Darius was murdered by Bessus in Parthia, and Alexander buried him with royal honours, acting, as Pierre Briant wrote, in many ways like the last of the Achaemenids.

From there he pushed into Central Asia, founding Alexandrias in places such as modern Kandahar and Alexandria Eschate in Tajikistan. In Sogdiana and Bactria he fought Spitamenes, took the Scythians at the Jaxartes, and executed Bessus after his betrayal. Yet success began to curdle. Alexander adopted Persian dress and proskynesis, demanded gestures the Macedonians reserved for gods, and alienated men who had followed him from the start. Philotas was executed for failing to warn him of a plot, Parmenion was killed at Ecbatana, and at Maracanda he slew Cleitus the Black in drunken fury after Cleitus accused him of abandoning Macedonian ways.