The battle of Sardis (498 BC) was a minor success for the Greeks during the Ionian Revolt, and despite being followed by a retreat and a defeat at Ephesus, helped to spread the revolt to Byzantium, the Hellespont and Caria.
In 499 the Ionian cities rebelled against Persian rule, led by Aristagoras of Miletus. The rebels overthrew the Persian-supported tyrants, appointed generals, and then attempted to find allies in Athens. Aristagores travelled to Greece, where he was turned down by the Spartans. He had more success in Athens, which agreed to send twenty ships (nearly half of its fleet at that time). Eretria also sent five ships to repay a debt to Miletus. This limited assistance arrived in Ionian in the spring of 498.
Aristagores realised that the rebels needed a major success to establish their cause. He decided to attack Sardis, the capital of the satrapy of Lydia and the base of Aristagores's former ally Artaphernes.
Aristagores appointed his brother Charopinus and Hermophantus of Miletus to command this expedition. Most of the troops came from Miletus, although the Athenians and Eretrians also took part, as did smaller contingents from the other Ionian cities. The army sailed to Coresus in the territory of Ephesus, where they collected local guides and began the march on Sardis.
The march on Sardis was unopposed. When they reached the city Artaphernes was unable to defend the main city, and retreated to the acropolis with his men. The Ionians entered the city, but before they could loot it a fire broke out and soon spread across the city. This forced the Persians to leave the acropolis and take shelter by the banks of the River Pactolus, which ran through the middle of the city's main square.
Ionian morale can't have been very high, for when they saw these troops preparing to make a stand by the river they abandoned the burning city and retreat to nearby Mt. Tmolus. That night they began to retreat back towards their fleet.
News of the attack triggered an effective Persian response. Troops from the surrounding provinces rushed towards Sardis. They arrived after the Greeks had begun their retreat, but then followed them back towards Ephesus, where they inflicted a major defeat on the Greek force. In the aftermath of this defeat the Ionian army dissolved into its individual components, and the rebel cities rarely acted together on land after this. The Athenians decided to withdraw their support for the war.
The sack of Sardis did achieve its main aim, of spreading the revolt. It helped convince Byzantium and number of Greek cities in the Hellespont, as well as the Carians, to join the revolt.