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The Persian cavalry, which threatened the Athenian infantry, was embarked to row around and capture Athens while its army was facing off against the Persian infantry. The lightly armed Persian infantry was inferior to Athenian armoured infantry and when the cavalry left, the Athenians took the opportunity to rush down and defeat it.
So take your pick – the failure of the Persian gamble with their cavalry; the initiative of the Athenians taking the chance in defeating the unsupported Persian infantry; the stamina of the Athenian infantry in running back the 26 miles to head off the Persian cavalrt when it arrived at the city. All most important and hard to separate out the ‘mostest’.