• The Thirty Tyrants, an oligarchy set up by Lysander to rule Athens after her defeat at the hands of Sparta in the Peloponnesian War, were defeated by Thrasybulus at the Battle of Piraeus.
• The king of China's declining Zhou dynasty officially recognizes the separatist states of Wei, Han, and Zhao, an event historians now use to mark the beginning of an anarchic era in Chinese history known as the Warring States Period.
• The Athenians officially adopt the Ionian alphabet, which became the basis of the modern Greek alphabet and, via the Romans, all the modern Roman alphabets. Previously, there had been several competing alphabet systems in use in the Greek world.
• Dionysius, the tyrant of Syracuse, conquers the Sicilian city of Taormina, captures Catania, and destroys the colony at Naxos.
• Construction begins on the earliest version of the Great Wall of China, which was actually several separate walls.

These people died in 403 BC:

• Critias, Athenian philosopher, a relative and friend of Plato, and one of the cruelest of the Thirty Tyrants, in the battle at Piraeus.

404 BC - 403 BC - 402 BC

5th century BC