Why is the battle of marathon known as the single most important battle in greek history

It was freedom, philosophers argued, that had at Marathon been defended by the Athenians. Because their victory had inspired other Greeks to resist Xerxes, Marathon had been an important battle: in Marathon, the foundations of western civilization had been laid.

The Battle of Marathon was significantbecause it proved to the Greeks that the Persians were not 'invincible', which boosted the moral of the Greek troops, increasing their confidence to incline themselves in a common cause if the Persian attacked again (which they would).