The First Persian War
In around 492 BC, a large number of members of the colony in Thracy returned to Athens, as the Persian army that Darius had left there after retreating from Scythes in southern Russia, started their attempt to conquer the colony.
However, before Darius had reached the borders of Greece , his fleet was heavily damaged by a huge storm in Athos. With the lose of reinforcements and supplies, and the attack by a tribe of nomads on his army, Darius aborted the attempt.
For his second attempt of his expedition, Darius used a smaller fleet, as he believed that the previous failure was due to a lack of co-operation between his army and fleet. This fleet crossed the Aegean in 490 BC and gained control of a small number of Greek islands.
This led to Athens believing that Darius’ expedition was in fact revenge, for their and Eretria’s involvement in Ionic revolt.
More likely however, is that this was a follow up to the failure of the 492 BC attempt. The capture of control over the Aegean was vital to the Persians in order to expand their empire.