Battle of Marathon

On September 12, 490 BC, the traditional date of the Battle of Marathon where a small Athenian force defeated the Persian Empire, took place. An excerpt from the article:

"The Battle on the plain of Marathon in September 490 BCE between Greeks and the invading forces of Persian king Darius I (r. 522-486 BCE) was a victory that would go down in folklore as the moment the Greek city-states showed the world their courage and excellence and won their liberty. Although in reality the battle only delayed the Persians in their imperialistic ambitions and greater battles would follow, Marathon was the first time that mighty Persian Achaemenid Empire had been shown beatable and the battle would be represented in Greek art - literature, sculpture, architecture, and pottery - as a crucial and defining moment in the history of Greece.

The Persian Empire
Persia, under the rule of Darius I, was already expanding into mainland Europe and had subjugated Ionia, Thrace, and Macedonia by the beginning of the 5th century BCE. Next in King Darius' sights were Athens and the rest of Greece. Just why Greece was coveted by Persia is unclear. Wealth and resources seem an unlikely motive; other more plausible suggestions include the need to increase the prestige of the king at home or to quell once and for all a collection of potentially troublesome rebel states on the western border of the empire. The Ionian rebellion, the symbolic offering of earth and water in submission to the Persian satrap in 508 BCE, and the attack by Athens and Eretria on the city of Sardis in 499 BCE had not been forgotten either.

The invaders would meet their match in 490 BCE when the Greek forces led by Athens gathered at the plain of Marathon.
Whatever the exact motives, in 491 BCE Darius once again sent envoys to call for the Greeks' submission to Persian rule. The Greeks sent a no-nonsense reply by executing the envoys, and Athens and Sparta promised to form an alliance for the defence of Greece. Darius' response to this diplomatic outrage was to launch a naval force of 600 ships and 25,000 men to attack the Cyclades and Euboea, leaving the Persians just one step away from the rest of Greece. However, the invaders would meet their match in 490 BCE when the Greek forces led by Athens gathered at the plain of Marathon to defend their country from foreign subjugation."