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The invasion then sparked what ultimately developed to be the Greco-Persian wars of the 5th century BCE, where Athens played a leading role among the Greek city-states involved. The initial beginning of this war in 492-490 went well for the Persians, as they destroyed the Greek city of Eretria and captured territory in the Cyclades and Thrace. However, at the Battle of Marathon a force of about 10,000 Athenians and Plataeans were able to defeat the Persians, perhaps the first significant victory of the Greeks against the Persians. This created a setback for the Persians as they could not invade Greece for another 10 years. The second invasion of Greece by the Persian army and fleet was much larger, forcing Athens to create alliances, in particular with Sparta. Initially the Persian force was successful at the battle of Thermopylae against the Greek allies, although possibly at great loss. After the battle of Plataea, the Persians were defeated and were forced to retreat once again. From this point Athens and the Greek allies were on the offensive and this begins Athens' leadership in an alliance that eventually formed the Delian league, which continued the wars against the Persians, mostly now in Egypt and Cyprus.

==Leader of the Delian League==