Battle of Marathon took place during the first Persian invasion of Greece. Although the Persian had advantage (26,000 against 11,000), Greeks chose wisely the spot to confront the enemy - area full of marshes and mountains that gave Persians no chance to use their most powerful weapon, cavalry. Second important factor was deployment of the forces - while the Persians always used their best warrior in th middle of the formation Greek deployed their strongest units on the flanks and thus easily broke through the enemy lines.
It was great victory for Greeks - weak Persin flanks broke into panic and begun to retreat towards their ships. This battle ended the first invasion of the Greece. The legend of the Marathon run gave the start to the Olympic discipline that is still considered limit to the human body - according to legends, an Athenian runner named Pheidippides, was sent after the battle back to Athens to pass the information about the victory. He have run for 42 km and died after informing the Athenians about the victory, but according to modern historians it is just a legend.