331 BC

By place

Macedonia

• Alexander of Macedonia departs from Egypt and leads his forces towards Phoenicia. He leaves Cleomenes of Naucratis as the ruling nomarch to control Egypt.
• October 1 – Alexander is victorious in the Battle of Gaugamela over the Persian King Darius III. Alexander pursues the defeated Persian forces to Arbela, Darius moves his Bactrian cavalry and Greek mercenaries into Media.
• Alexander encounters for the first time war elephants after the battle in Darius' camp. In the capital, Susa, Alexander gains access to huge treasures amounting to 50,000 gold talents.

  Greece

• While Alexander is fighting in Asia, Agis III of Sparta, profiting from the Macedonian king's absence from Greece, leads some of the Greek cities in a revolt. With Persian money and 8,000 Greek mercenaries, he holds Crete against Macedonian forces. In the Peloponnesus he routs a force under the Macedonian general Coragus and, although Athens stays neutral, he is joined by Elis, Achaea and Arcadia, with the exception of Megalopolis, the staunchly anti-Spartan capital of Arcadia, which Agis III's forces besiege.
• Alexander's regent Antipater leads the Macedonians to victory over King Agis III in the Battle of Megalopolis.

  Italy

• Alexander of Epirus takes Heraclea from the Lucanians, and Terina and Sipontum from the Bruttii.
• Tarentum turn against Alexander of Epirus when they realize that he intends to create a kingdom of his own in southern Italy. Alexander is defeated and killed in the Battle of Pandosia on the banks of the Acheron.

  Roman Republic

• The Gallic tribe of the Senones and the Romans conclude a peace and enter upon a period of friendly relations which lasts the rest of the century.