The Persians unloaded their cavalry before the battle started because they expected the Greeks to surrender without fighting—so they didn't think they'd need mounted troops at all.
We know this happened because there's zero mention of Persian horses in any of the actual combat at Marathon. For an army famous for cavalry tactics, that's not an accident. It's a choice. And that choice tells us something crucial about Persian assumptions going into this battle.
Everyone pictures this dramatic moment where Greek soldiers charge across the plain and catch the Persians off guard. But the real story is weirder and more interesting. The Persians had already made their bet before the fighting even started. They bet that most Greek cities would just hand over earth and water—the Persian way of saying "you're ours now." Darius sent envoys demanding exactly this around 491 BCE, just a year before Marathon. That's not a vague threat. That's an ultimatum with a clock on it.
When the Persians landed at Marathon, they probably expected Athens to do what sensible cities were doing: negotiate, submit, live to fight another day. Cavalry would be useful if the Greeks ran. Cavalry would be useful for pursuit. But if you're expecting your enemy to lay down without a real fight, why keep your best shock troops loaded on ships?
Herodotus, Histories 7.32 (fifth century BCE): The earliest detailed account of Persian military preparations and their expectations of Greek submission before Marathon.
Herodotus is our oldest source here, and he's clear that Persian envoys had been demanding submission from Greek cities. The absence of cavalry in the Marathon fighting itself—mentioned across multiple ancient accounts—is what really gives away the Persian strategy.
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