They got written out almost immediately because the Spartans won the propaganda war before the bodies were even buried. Seven hundred Thespians stayed and died alongside the 300 Spartans—that's their entire military-age population—but within months, Simonides was already carving the famous epitaph: "Go tell the Spartans." The legend wasn't organic. It was deliberate, crafted while Athens and Sparta were still technically allies, and it was designed to make Sparta look like the heroes.
Herodotus, writing around 430 BCE, does mention both groups fought and died at Thermopylae. He names the Thespians specifically—says they voluntarily stayed when given the chance to leave. But here's the problem: once Simonides and later poets got hold of the story, they focused entirely on Spartan discipline and honor. The Thespians were fighting for their survival. Their city never recovered from losing an entire generation. But nobody remembers that part.
Herodotus, Histories Book VII, 225 (c. 430 BCE)
We think of Thermopylae as a timeless story that naturally became legendary. Wrong. This was instant myth-making. The propaganda started before the war even ended. We only remember 300 because Sparta had better PR, not because they were the only ones who fought. The Thebans actually got accused of being cowards in later retellings, which is its own kind of erasure. Three hundred died. Seven hundred died. But somehow only one number stuck.
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