Extract the answer to the question from the following context. Question: Which doings of Alexander III's did not withstand the test of time? Context: Following the assassination of Phillip II, his son Alexander III ("The Great") assumed the leadership of the League of Corinth and launched an invasion of the Persian Empire with the combined forces of all Greek states in 334 BC. Undefeated in battle, Alexander had conquered the Persian Empire in its entirety by 330 BC. By the time of his death in 323 BC, he had created one of the largest empires in history, stretching from Greece to India. His empire split into several kingdoms upon his death, the most famous of which were the Seleucid Empire, Ptolemaic Egypt, the Greco-Bactrian Kingdom and the Indo-Greek Kingdom. Many Greeks migrated to Alexandria, Antioch, Seleucia and the many other new Hellenistic cities in Asia and Africa. Although the political unity of Alexander's empire could not be maintained, it resulted in the Hellenistic civilization and spread the Greek language and Greek culture in the territories conquered by Alexander. Greek science, technology and mathematics are generally considered to have reached their peak during the Hellenistic period.
the political unity of Alexander's empire