Athens and Eretria respond to the Ionianplea for help against Persia and send troops. An Athenian and Eretrian fleet transports Athenian troops to Ephesus. There they are joined by a force of Ionians and march upon Sardis, the capital of Artaphernes (the satrap of Lydia and brother to Darius I of Persia). Artaphernes, who has sent most of his troops to besiege Miletus, is taken by surprise. However, Artaphernes is able to retreat to the citadel and hold it. Although the Greeks are unable to take the citadel, they pillage the town and set fires that burn Sardis to the ground.
Retreating to the coast, the Greek forces are met by the Persians under Artaphernes and defeated in the Battle of Ephesus.
Having successfully captured several of the revolting Greekcity-states, the Persians under Artaphernes lay siege to Miletus. The decisive Battle of Lade is fought at the island of Lade, near Miletus' port. Although out-numbered, the Greek fleet appears to be winning the battle until the ships from Samos and Lesbos retreat. The sudden defection turns the tide of battle, and the remaining Greek fleet is completely destroyed. Miletus surrenders shortly thereafter, and the Ionian Revolt comes to an end.
The Persian leaders Artaphernes and Mardonius grant a degree of autonomy to the Ionian cities. They abstain from financial reprisals and merely exact former levels of tribute. The Persians abolish the Greek tyrannies in Ionia and permit democracies.
At the end of the military campaigns, the plebs retire to the Sacred Mountain outside Rome in the Secession of the Plebs. To end the succession, the plebeians gain acceptance from the patricians that they may choose two leaders to whom they give the title of Tribunes. The office of the tribunate is thereby established.
The aediles, magistrates of ancient Rome who are in charge of the temple and cult of Ceres, are first established. They are two officials of the plebeians, created at the same time as the tribunes, whose sanctity they share.
The Athenian people elect Themistocles as archon, the chief judicial and civilian executive officer in Athens. He favours resistance against the Persians.
Themistocles starts the construction of a fortified naval base at Piraeus, the port town of Athens.
Among the refugees arriving from Ionia after the collapse of the Ionian Revolt is a chief named Miltiades, who has a fine reputation as a soldier. Themistocles makes him a general in the Athenian army.
When Camarina, a Syracusan colony, rebels, Hippocrates, the tyrant of Gela, intervenes to wage war against Syracuse. After defeating the Syracusan army at the Heloros River, he besieges the city. However, he is persuaded by the intervention of forces from the Greek mainland city of Corinth to retreat in exchange for the possession of Camarina.
Darius I sends envoys to all Greek cities, demanding "earth and water for vassalage" which Athens and Sparta refuse.
The Greek city of Aegina, fearing the loss of trade, submits to Persia. The Spartan king, Cleomenes I tries to punish Aegina for its submission to the Persians, but the other Spartan king, Demaratus, thwarts him.
Cleomenes I engineers the deposing of Spartan co-ruler Demaratus (and his replacement by Cleomenes’ cousin Leotychidas) by bribing the oracle at Delphi to announce that this action was divine will. The two Spartan kings successfully capture the Persian collaborators in Aegina.
During a famine in Rome, Gaius Marcius Coriolanus advises that the people should not receive grain unless they would consent to the abolition of the office of tribune. For this, the tribunes have him condemned to exile. Coriolanus then takes refuge with the leader of the Volsci and during the following years they together lead the Volscian army against Rome, turning back only in response to entreaties from his mother and his wife.
Darius I sends an expedition, under Artaphernes and Datis the Mede across the Aegean to attack the Athenians and the Eretrians. Hippias, the aged ex-tyrant of Athens, is on one of the Persian ships in the hope of being restored to power in Athens.
When the Ionian Greeks in Asia Minor rebelled against Persia in 499 BC, Eretria joined Athens in sending aid to the rebels. As a result, Darius makes a point of punishing Eretria during his invasion of Greece. The city is sacked and burned and Darius enslaves its inhabitants. He intends the same fate for Athens.
September 12 – The Battle of Marathon takes place as a Persian army of more than 20,000 men is advised by Hippias to land in the Bay of Marathon, where they meet the Athenians supported by the Plataeans. The Persians are repulsed by 11,000 Greeks under the leadership of Callimachus and Miltiades. Some 6,400 Persians are killed at a cost of 192 Athenian dead. Callimachus, the war-archon of Athens, is killed in the battle. After the battle, the Persians return home.
Before the Battle of Marathon, the Athenians send a runner, Pheidippides, to seek help from Sparta. However, the Spartans delay sending troops to Marathon because religious requirements (the Carnea) mean they must wait for the full moon.
The Greek historian Herodotus, the main source for the Greco-Persian Wars, mentions Pheidippides as the messenger who runs from Athens to Sparta asking for help, and then runs back, a distance of over 240 kilometres each way. After the battle, he runs back to Athens to spread the news and raise the spirits. It is claimed that his last words before collapsing and dying in Athens are "Chairete, nikomen" ("Rejoice, we are victorious").
Hippias dies at Lemnos on the journey back to Sardis after the Persian defeat.
Cleomenes I is forced to flee Sparta when his plot against Demaratus is discovered, but the Spartans allow him to return when he begins gathering an army in the surrounding territories. However, by this time he has become insane, and the Spartans put him in prison. Shortly after, he commits suicide. He is succeeded as King of Sparta by a member of the Agiad house, his half-brother, Leonidas.