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Demaratus (Greek: Δημάρατος, Demaratos; Doric: Δαμάρατος, Damaratos) was a king of Sparta from around 515 BC to 491 BC. He was the 15th ruler of the Eurypontid dynasty and the firstborn son of King Ariston. During his reign, Demaratus is best known for his opposition to his co-ruler, King Cleomenes I of the Agiad dynasty. This rivalry ultimately led to his dethronement around 491 BC, following Cleomenes' accusations of illegitimacy and political maneuvering.
After his removal from power, Demaratus sought refuge in the Achaemenid Empire, where he was welcomed by King Darius I. He was granted land and cities in Asia Minor, integrating him into Persian society while retaining his influence. Demaratus served as an advisor to Xerxes I during the Second Persian invasion of Greece, providing strategic insights about Greek military tactics.