Alexander, born in Pella, the ancient capital of Macedonia, was the son of Philip II, king of Macedonia, and of Olympias, a princess of Epirus. Aristotle was Alexander's tutor; he gave Alexander a thorough training in rhetoric and literature and stimulated his interest in science, medicine, and philosophy. In the summer of 336 bc Philip was assassinated, and Alexander ascended to the Macedonian throne. He found himself surrounded by enemies at home and threatened by rebellion abroad. Alexander disposed quickly of all conspirators and domestic enemies by ordering their execution. Then he descended on Thessaly (Thessalia), where partisans of independence had gained ascendancy, and restored Macedonian rule. Before the end of the summer of 336 bc he had reestablished his position in Greece and was elected by a congress of states at Corinth.

In 335 bc as general of the Greeks in a campaign against the Persians, originally planned by his father, he carried out a successful campaign against the defecting Thracians, penetrating to the Danube River. On his return he crushed in a single week the threatening Illyrians and then hastened to Thebes, which had revolted. He took the city by storm and razed it, sparing only the temples of the gods and the house of the Greek lyric poet Pindar, and selling the surviving inhabitants, about 8,000 in number, into slavery. Alexander's promptness in crushing the revolt of Thebes brought the other Greek states into instant and abject submission.

Alexander began his war against Persia in the spring of 334 bc by crossing the Hellespont (modern Dardanelles) with an army of 35,000 Macedonian and Greek troops; his chief officers, all Macedonians, included Antigonus I, Ptolemy I, and Seleucus I. At the river Granicus, near the ancient city of Troy, he attacked an army of Persians and Greek mercenaries totaling 40,000 men. His forces defeated the enemy and, according to tradition, lost only 110 men; after this battle all the states of Asia Minor submitted to him (see Battle of Granicus). In passing through Phrygia he is said to have cut with his sword the Gordian knot.

Continuing to advance southward, Alexander encountered the main Persian army, commanded by King Darius III, at Issus, in northeastern Syria. The size of Darius's army is unknown; the ancient tradition that it contained 500,000 men is now considered a fantastic exaggeration. The Battle of Issus, in 333, ended in a great victory for Alexander. Cut off from his base, Darius fled northward, abandoning his mother, wife, and children to Alexander, who treated them with the respect due to royalty. Tyre, a strongly fortified seaport, offered obstinate resistance, but Alexander took it by storm in 332 after a siege of seven months. Alexander captured Gaza next and then passed on into Egypt, where he was greeted as a deliverer. By these successes he secured control of the entire eastern Mediterranean coastline. Later in 332 he founded, at the mouth of the Nile River, the city of Alexandria, which later became the literary, scientific, and commercial center of the Greek world. Cyrene, the capital of the ancient North African kingdom of Cyrenaica, submitted to Alexander soon afterward, extending his dominion to Carthaginian territory.

In the spring of 331 Alexander made a pilgrimage to the great temple and oracle of Amon-Ra, Egyptian god of the sun, whom the Greeks identified with Zeus. The earlier Egyptian pharaohs were believed to be sons of Amon-Ra; and Alexander, the new ruler of Egypt, wanted the god to acknowledge him as his son. The pilgrimage apparently was successful, and it may have confirmed in him a belief in his own divine origin.

Turning northward again, he reorganized his forces at Tyre and started for Babylon with an army of 40,000 infantry and 7,000 cavalry. Crossing the Euphrates and the Tigris rivers, he met Darius at the head of an army of unknown size, which, according to the exaggerated accounts of antiquity, was said to number 1 million men; this army he completely defeated in the Battle of Gaugamela, on October 1, 331 bc. Darius fled as he had done at Issus and was later slain by one of his own satraps. Babylon surrendered after Gaugamela, and the city of Susa with its enormous treasures was soon conquered. Then, in midwinter, Alexander forced his way to Persepolis, the Persian capital. After plundering the royal treasuries and taking other rich booty, he burned the city during a drunken binge. His domain now extended along and beyond the southern shores of the Caspian Sea, including modern Afghanistan and Baluchistan, and northward into Bactria and Sogdiana, the modern Western Turkistan, also known as Central Asia. It had taken Alexander only three years, from the spring of 330 bc to the spring of 327 bc, to master this vast area.

In order to complete his conquest of the remnants of the Persian Empire, which had once included part of western India, Alexander crossed the Indus River in 326 bc, and invaded the Punjab as far as the river Hyphasis (modern Beās); at this point the Macedonians rebelled and refused to go farther. He then constructed a fleet and passed down the Indus, reaching its mouth in September 325 bc. The fleet then sailed to the Persian Gulf. With his army, he returned overland across the desert to Media. Shortages of food and water caused severe losses and hardship among his troops. Alexander spent about a year organizing his dominions and completing a survey of the Persian Gulf in preparation for further conquests. He arrived in Babylon in the spring of 323 bc. In June he contracted a fever and died. He left his empire, in his own words, “to the strongest”; this ambiguous testament resulted in dire conflicts for half a century.

Alexander was one of the greatest generals of all time, noted for his brilliance as a tactician and troop leader and for the rapidity with which he could traverse great expanses of territory. He was usually brave and generous, but could be cruel and ruthless when politics demanded. The theory has been advanced that he was actually an alcoholic having, for example, killed his friend Clitus in a drunken fury. He later regretted this act deeply. As a statesman and ruler he had grandiose plans; according to many modern historians he cherished a scheme for uniting the East and the West in a world empire, a new and enlightened “world brotherhood of all men.” He trained thousands of Persian youths in Macedonian tactics and enrolled them in his army. He himself adopted Persian manners and married Eastern wives, namely, Roxana (died about 311 bc), daughter of Oxyartes of Sogdiana, and Barsine (or Stateira; died about 323 bc), the elder daughter of Darius; and he encouraged and bribed his officers to take Persian wives. Shortly before he died, Alexander ordered the Greek cities to worship him as a god. Although he probably gave the order for political reasons, he was, in his own view and that of his contemporaries, of divine birth. The order was largely nullified by his death shortly after he issued it.

To bind his conquests together, Alexander founded a number of cities, most of them named Alexandria, along his line of march; these cities were well located, well paved, and provided with good water supplies. Greek veterans from his army settled in them; young men, traders, merchants, and scholars were attracted to them; Greek culture was introduced; and the Greek language became widely known. Thus, Alexander vastly extended the influence of Greek civilization and prepared the way for the kingdoms of the Hellenistic period and the conquests of the Roman Empire.