Alexander III of Macedon became king at twenty, in 336 BCE, when his father Philip — who had already drilled Macedon into the most lethal army in the Greek world — was assassinated. Alexander died at thirty-two, in Babylon, having conquered an empire stretching from the Adriatic to the Indus. He fought no losing battles — Granicus, Issus, Gaugamela, Hydaspes, each against larger forces. He founded perhaps seventy cities, most of them named Alexandria after himself. He died of fever, or perhaps poison, with no plan for succession and an infant heir, and his generals — the Diadochi — carved the empire into rival kingdoms and fought one another for forty years. The territorial conquest did not survive him. Something else did.
What Alexander permanently installed was the Greek language as a working medium of administration, commerce, and high culture from Egypt to Bactria. He carried it not by settlement but by infrastructure: each Alexandria was a garrison-colony of Greek and Macedonian veterans, a node of language, coinage, and gymnasium into which ambitious locals bought. The kingdoms his generals founded — the Ptolemies in Egypt, the Seleucids across Asia, the Antigonids in Macedon — competed not only in war but in patronage, and that rivalry funded the science. For the next three centuries — the Hellenistic age — an educated person from Marseille to Kabul could read the same books, study at the great libraries of Alexandria or Pergamum, worship recognizably similar gods, and use a common coinage. Koine Greek became the lingua franca of the eastern Mediterranean for a thousand years; it was the original language of the New Testament, which is why a Galilean carpenter's teachings reached Rome at all. In Afghanistan, the Greco-Buddhist sculptors of Gandhara gave the Buddha his first human face, draped in the folds of a Greek himation. The Hellenistic synthesis — Greek thought meeting Egyptian, Babylonian, Persian, and Indian traditions — produced Euclid's geometry, Archimedes' mechanics, Eratosthenes' measurement of the Earth, and the astronomy later gathered by Ptolemy, the intellectual substrate of late antiquity and, through Arabic translation in Baghdad's House of Wisdom, of the medieval revival that fed back into Europe. Conquest opened the channel; what flowed through it was knowledge.
Alexander is the prototype of cultural conquest by infrastructure rather than by demography. The Macedonians never numerically replaced the populations they overran; they installed a new operating system — a language, a script, a coinage, a network of cities — and let local elites adopt it because it was useful. Every later linguistic empire has been read against this template: Latin in the Roman west, Arabic after Islam, English after 1945, now the half-American digital layer through which the planet types. The deepest conquests change not who is in the room but what language they think in.