The battle of Ephesus (498 BC) was a victory won by the Persians over a rebellious Greek army that was retreating from an attack on the city of Sardis (Ionian Revolt).
The Ionian rebels began the campaigning season of 498 with an attack on Sardis, the capital of the satrapy of Lydia. This wasn't a great success. The largely Milesian army, with an Athenian contingent, reached the city unopposed, but the Persians withdrew into the acropolis. The city then caught fire, and the Persians were forced to try and make their way to the River Pactolus. The Ionians retreated when it was clear that the Persians were about to put up a fight, and pulled back to Mt. Tmolus. That night they began to retreat back to their fleet.
The Persians responded quickly to the attack, and a relief force was raised from the provinces west of the River Halys. By the time the Persians reached Sardis, the Greeks had already gone, but the Persians were able to follow their trail, which led back to Ephesus.
The Persians caught up with the Greeks near to Ephesus. The Greeks had enough time to form up, but they suffered a heavy defeat in the ensuing battle. Amongst the dead was Eualcides, commander of the Eretrian contingent, a famous athlete.
In the aftermath of the battle the surviving Ionians split up and went back to their individual cities. The Athenians decided to withdraw their support for the revolt, and also returned home. Aristagoras, the leader of the revolt, made several attempts to win them back to his side, but without success.
Although the expedition to Sardis had ended in defeat, the sack of the city was a great encouragement for the rebels. It helped encourage Byzantium and a number of Greek cities on the Hellespont to join the revolt, as well as most of Caria and Caunus.
The defeat at Ephesus ended the last major attempt by the rebels to coordinate their activities inland. Their main effort for the rest of the war would be carried out at sea or close to the shore, where they could use their fleets to good effect.