According to the Encyclopedia Britannica, Alexander the Great's major contribution to history was the spread of Greek culture throughout the Middle East and Central Asia. His large territorial empire also encouraged trade between cultures that had previously had little contact, encouraging economic growth and the flow of ideas between Greece and North India.
Historian Johann Gustav Droysen called the spread of Greek culture that happened in the wake of Alexander's conquest "Hellenization." Alexander encouraged Hellenization by founding at least 70 new cities throughout his empire and populating them with Greeks. Though the empire that Alexander gained through his military triumph split into various kingdoms after his early death in 323 B.C. at the age of 32, the Greek generals who ruled those kingdoms, who were called the Diadochi, still promoted Greek culture. The spread of Hellenistic ideas bound together the cultures of the vast areas of his empire, giving them a lingua franca, koin? Greek, and a shared set of cultural touchstones that made communication and trade easier.
The Enyclopedia Brittanica also credits Alexander with the eventual rise of the Roman and Byzantine empires and the spread of Christianity throughout the ancient world. These political and religious systems used the Greek-speaking infrastructure of the world that Alexander joined together to project influence over huge distances, a feat that would have been much more difficult without the ground that Alexander's conquests had prepared. His conquests also increased scientific and geographical knowledge of the ancient world.