The Anopaea was a mountain trail that ran behind the Greek defensive line at Thermopylae, and the Persians learned about it on the second day of fighting when a local man named Ephialtes betrayed its exact location to Xerxes.
Here's the thing that gets messy when you dig into the sources: Leonidas probably knew about the Anopaea from the start—Herodotus claims the Greeks were aware of the route before they even arrived. But knowing a path exists and knowing exactly where it is are two completely different problems. Ephialtes showed up around day two with specific details about how the Persians could use it to flank the Greek position.
This timing matters because it tells us something important about Leonidas's original plan. He wasn't necessarily setting up for a glorious last stand from day one. He was actually trying to hold the pass until the moment he learned the Persians had found a way around him.
"Herodotus says they knew about the Anopaea path from the beginning, but Ephialtes only betrayed its exact location on the second day of fighting."
We remember the 300 Spartans. We barely remember the 700 Thespians who chose to stay and fight beside them—which was literally their entire military-age male population. Their city never recovered from that decision. The flanking route wasn't some impossible-to-predict disaster either. It was a known vulnerability that became fatal the moment someone told the Persians exactly how to use it.
Herodotus, Histories Book VII, 212-214 and 225 (c. 430 BCE) — Our oldest account describing the Anopaea path, the Greek knowledge of it, and Ephialtes's betrayal during the second day of battle.
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