Which Arcadian city-state sent 120 hoplites to Thermopylae?

The Arcadian city-state that sent 120 hoplites to Thermopylae was Orchomenos. Its contingent joined the wider Greek force assembled to block Xerxes I's Persian invasion at the narrow pass in 480 BC.

The 120 men from Orchomenos were part of a larger Arcadian contribution. Herodotus records that Mantinea sent 500 hoplites, Tegea sent another 500, and other Arcadian communities supplied about 1,000 more. These figures show that the famous Spartan 300 were only one small part of the allied Greek army defending Thermopylae.

Orchomenos was an ancient city in Arcadia in the central Peloponnese, distinct from Orchomenos in Boeotia. That geographic distinction is a common source of confusion in ancient Greek history. The Arcadian troops fought alongside Spartans, Thespians, Thebans, Phocians, Corinthians, and other allies.

The Greek position was eventually outflanked after the Persian army learned of the Anopaea mountain path. Most of the allied force retreated, while a smaller rearguard remained. The 120 figure identifies Orchomenos specifically; it should not be confused with the larger contingents from Tegea or Mantinea.