Demaratus (Greek: Δημάρατος Demaratos; Doric: Δαμάρατος Damaratos) was a king of Sparta from around 515 BC to 491 BC. The 15th of the Eurypontid line, he was the first son born to his father, King Ariston.
As king, Demaratus is known chiefly for his opposition to the co-ruling Spartan king, Cleomenes I. He later fled to Achaemenid Persia, where he was given asylum and land, and fought on the Persian side during the Second Persian invasion of Greece.
Demaratus, the son of King Ariston (r. c.550–c.515), belonged to the Eurypontid dynasty, one of the two royal families of Sparta (the other being the Agiads). After Ariston had remained childless from his first two wives, he took the wife of Agetus, one of his friends. Less than 10 months later, Demaratus was born, but Ariston rejected his paternity before the ephors. He nonetheless changed his mind later and recognised Demaratus as his son, who succeeded him at his death around 515.
When Cleomenes attempted to make Isagoras tyrant in Athens, Demaratus tried to frustrate the plans. In 491 BC, Aegina was one of the states that gave the symbols of submission (earth and water) to Persia. Athens at once appealed to Sparta to punish that act of medism, and Cleomenes I crossed over to the island to arrest those responsible. His first attempt was unsuccessful because of interference from Demaratus, who did his utmost to bring Cleomenes into disfavour at home.
In retaliation, Cleomenes urged Leotychidas, a relative and personal enemy of Demaratus, to claim the throne on the grounds that the latter was really the son not of Ariston but of Agetus, his mother's first husband. Cleomenes bribed the Delphic oracle to pronounce in favour of Leotychidas, who became king in 491 BC.
After the deposition of Demaratus, Cleomenes visited the island of Aegina for a second time. Accompanied by his new colleague, Leotychides, he seized ten of the leading citizens and deposited them at Athens as hostages.
On his abdication, Demaratus was forced to flee. He went to the court of Persian King Xerxes I, who gave him the cities of Teuthrania and Halisarna, around Pergamum, where his descendants Eurysthenes and Procles still ruled in the early 4th century BC.
Demaratus accompanied Xerxes I on his invasion of Greece in 480 BC and is alleged to have warned Xerxes not to underestimate the Spartans before the Battle of Thermopylae:
The same goes for the Spartans. One-against-one, they are as good as anyone in the world. But when they fight in a body, they are the best of all. For though they are free men, they are not entirely free. They accept Law as their master. And they respect this master more than your subjects respect you. Whatever he commands, they do. And his command never changes: It forbids them to flee in battle, whatever the number of their foes. He requires them to stand firm – to conquer or die. O king, if I seem to speak foolishly, I am content from this time forward to remain silent. I only spoke now because you commanded me to. I do hope that everything turns out according to your wishes.— Herodotus vii (trans. G. Rawlinson)
Xerxes also asked Demaratus about his knowledge of the Greeks and if they would put up a fight against the Persian army. In response, Demaratus spoke favourably about the Greeks even after he had been deposed and exiled from Sparta:
So Demaratus said, 'my lord, you have asked me to tell the whole truth—the kind of truth that you will not be able to prove false at a later date. There has never been a time when poverty was not a factor in the rearing of the Greeks, but their courage has been acquired as a result of intelligence and the force of law. Greece has relied on this courage to keep poverty and despotism at bay. I admire all the Greeks who live in those Dorian lands, but I shall restrict what I have to say to the Lacedaemonians alone. First, then, there's no way in which they will ever listen to any proposals of yours which will bring slavery on Greece; second, they will certainly resist you, even if all the other Greeks come over to your side. As for the size of their army, there's no point in your asking how, in terms, of numbers, they can do this. If there are in fact only a thousand men to march out against you (though it may be fewer or it may be more), then a thousand men will fight you.'— Herodotus vii (trans. Robin Waterfield)
Demaratus was one of several Greeks aristocrats who took refuge in the Achaemenid Empire after reversals at home. Other famous cases were Themistocles and Gongylos. In general, they were generously rewarded by the Achaemenid kings, received land grants to support them and ruled over various cities in Asia Minor.
Demaratus's family continued to flourish in Asia as subjects of the Persians, and several of his descendants have been identified. One of them was likely Demaratus, the son of Gorgion, who was restored to Sparta in the early 3rd century BC and was in turn the putative great-grandfather of Nabis, the last king of Sparta (ruled 207–192).