Battle of Marathon

Ten years before the famous Spartan "Last Stand" (Battle of Themopylae - Leonidas' 300 heroic army), the Persian empire had already made a futile attempt to enslave Mainland Greece.
Marathon, a city state near Athens, was the perfect setting for this early stand.

Video coming from The History Channel - Battle of Marathon

The year is 490 BC; the Greco Persian Wars had already started and King Darius (Xerxes' father) is leading the Asian army to subjugate the Hellenic city-states. After a series of victories, the Persians arrive at the shores of Marathon, where they are "received" by general Miltiades and his Athenian hoplites. Though the Spartans were early invited to hold the Persian invasion side by side with the Athenians, they only arrived late after the Persians had already fled back to Asia.

Ten years later Darius' heir, Xerxes, was back again in Greece to try to finish what his father had already started. After an initial success in Thermopylae and Artemisium (480 BC), the Persians were later defeated at Salamis and Plataea (479 BC), giving a final victory to the Greeks and expelling the Persian Empire from their territory.

The History Channel, once again, has made a good job at delivering this part of history to a wide audience. We can argument that it's a bit "cinematographic" and "mythological", putting Miltiades as the strategic saviour of all Greece and other details to take into consideration. Nevertheless, their job is great; an entertaining must see !

The iconic Marathon race (Wikipedia modified version)

According to the Herodotus, father of History, an Athenian runner named Pheidippides (or Philippides in some accounts) was sent to run from Athens to Sparta to ask for assistance before the battle, covering the distance of 140 miles in two days. Then, following the battle, the Athenian army marched the 25 or so miles back to Athens at a very high pace (considering the quantity of armour, and the fatigue after the battle), in order to head off the Persian force sailing around Cape Sounion. They arrived back in the late afternoon, in time to see the Persian ships turn away from Athens, thus completing the Athenian victory.

Later, in popular imagination, these two events became confused with each other, leading to a legendary, but inaccurate version of events. This myth has Pheidippides running from Marathon to Athens after the battle, to announce the Greek victory with the word "Nenikēkamen!" (We were victorious!), whereupon he promptly died of exhaustion.

When the idea of a modern Olympics became a reality at the end of the 19th century, the initiators and organizers were looking for a great popularizing event, recalling the ancient glory of Greece. The idea of organizing a 'marathon race' came from Michel Bréal, who wanted the event to feature in the first modern Olympic Games in 1896 in Athens. This idea was heavily supported by Pierre de Coubertin, the founder of the modern Olympics, as well as the Greeks. This would echo the legendary version of events, with the competitors running from Marathon to Athens. So popular was this event that it quickly caught on, becoming a fixture at the Olympic games, with major cities staging their own annual events. The distance eventually became fixed at 26 miles 385 yards, or 42.195 km, though for the first years it was variable, being around 25 miles (40 km) — the approximate distance from Marathon to Athens.