The battle of Eretria (490 BC) was the second and final Persian success during the campaign that ended in defeat at Marathon. During the Ionian Revolt Athens and Eretria on Euboea had offered some support to the rebels. Darius I was determined to take revenge on the Greek cities, and in 492 he sent an army along the land route through Thrace. This expedition, commanded by his son-in-law Mardonius, restored Persian control over Thrace and forced the Macedonians to submit, but the fleet was then destroyed in a storm while sailing around Mt Athos and Mardonius was forced to retreat (Greco-Persian Wars).
After this setback Darius ordered the construction of a fleet of horse transports. In 490 he raised a new army, and placed Datis the Mede and Artaphrenes son of Artaphernes, a nephew of Darius, in command of the expedition. This time the Persians planned to use the sea route across the Aegean. They left Samos and crossed the sea via Icaria, Naxos and Delos. They then landed at the eastern end of Euboea, where they were held up for a period by the refusal of Carystus to submit. After a short siege Carystus surrendered, and the Persians sailed around the Euboean coast, landing at Tamynae, Choereae and Aegilia, east of the city.
While the Persians had been crossing the Aegean, the Eretrians had asked for help from Athens, and debated how to defend their city. The Athenians offered them 4,000 men from Chalcis. The debate was less clear-cut. One faction wanted to retreat into the Euboean hills. Another wanted to defend the city. A third wanted to surrender to the Persians.
As a result of this confusion the Athenian contingent decided to return to the mainland, possibly following advice from Aeschines, son of Nothon, one of the Eretrian leaders.
At Eretria the faction that had decided to defend the city won the debate. According to Herodotus a six day long battle raged, either outside the city or as a siege with the Eretrians defending the walls. He describes their plan as to meet the Persians in battle outside the city and to defend their walls, so either is possible.
The city finally fell because of treachery on the part of two Eretrian leaders, Euphorbus son of Alcimachus and Philagrus son of Cyneas.
The Persians sacked Eretria, destroying the religious sanctuaries. They justified this as revenge for the destruction of the sanctuaries at Sardis in 498 during the Ionian Revolt, although this may well have been accidental. The population of Eretria was enslaved, although when they finally arrived in Persia Darius is said to have relented and settled them at Cissia, quite close to Susa.
The Persians rested for a few days after the fall of Eretria, and then turned south and sailed across to the mainland, landing at Marathon, in the north-east of Attica. The Athenians reacted by rushing their army to Marathon, where they went on to inflict a heavy defeat on the Persians. The Persians made a brief attempt to attack Athens directly, but then retreated back across the Aegean.