Greco-Persian Wars
Sparta

The Spartan king Demaratus had been stripped of his kingship in 491 BC, and replaced with his cousin Leotychides. Sometime after 490 BC, the humiliated Demaratus had chosen to go into exile, and had made his way to Darius's court in Susa. Demaratus would from then on act as an advisor to Darius, and later Xerxes, on Greek affairs, and accompanied Xerxes during the second Persian invasion. At the end of Herodotus's book 7, there is an anecdote relating that prior to the second invasion, Demaratus sent an apparently blank wax tablet to Sparta. When the wax was removed, a message was found scratched on the wooden backing, warning the Spartans of Xerxes's plans. However, many historians believe that this chapter was inserted into the text by a later author, possibly to fill a gap between the end of book 7 and the start of book 8. The veracity of this anecdote is therefore unclear.
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