From Herodotus..The Histories.
Xerxes had sent heralds to Sparta to advise them of his intention to invade and offer surrender terms. The Spartans threw the heralds in a pit to die. This brought a curse upon them as heralds traveled under something akin to a white flag which should have gotten them safe passage.
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2] Now there was a long period after the incident I have mentioned above during which the Spartans were unable to obtain good omens from sacrifice. The Lacedaemonians were grieved and dismayed by this and frequently called assemblies, making a proclamation inviting some Lacedaemonian to give his life for Sparta. Then two Spartans of noble birth and great wealth, Sperthias son of Aneristus and Bulis son of Nicolaus, undertook of their own free will to make atonement to Xerxes for Darius' heralds who had been killed at Sparta
[3] Thereupon the Spartans sent these men to Media for execution.
Worthy of admiration was these men's deed of daring, and so also were their sayings. On their way to Susa, they came to Hydarnes, a Persian, who was general of the coast of Asia. He entertained and feasted them as his guests, and as they sat at his board, he asked:
2] “Lacedaemonians, why do you shun the king's friendship? You can judge from what you see of me and my condition how well the king can honor men of worth. So might it be with you if you would but put yourselves in the king's hands, being as you are of proven worth in his eyes, and every one of you might by his commission be a ruler of Hellas.”
[3] To this the Spartans answered: “Your advice to us, Hydarnes, is not completely sound; one half of it rests on knowledge, but the other on ignorance. You know well how to be a slave, but you, who have never tasted freedom, do not know whether it is sweet or not. Were you to taste of it, not with spears you would counsel us to fight for it, no, but with axes.”
This was their answer to Hydarnes. From there they came to Susa, into the king's presence, and when the guards commanded and would have compelled them to fall down and bow to the king, they said they would never do that. This they would refuse even if they were thrust down headlong, for it was not their custom, said they, to bow to mortal men, nor was that the purpose of their coming. Having averted that, they next said,
2] “The Lacedaemonians have sent us, O king of the Medes, in requital for the slaying of your heralds at Sparta, to make atonement for their death,” and more to that effect. To this Xerxes, with great magnanimity, replied that he would not imitate the Lacedaemonians. “You,” said he, “made havoc of all human law by slaying heralds, but I will not do that for which I censure you, nor by putting you in turn to death will I set the Lacedaemonians free from this guilt.”
This conduct on the part of the Spartans succeeded for a time in allaying the anger of Talthybius, in spite of the fact that Sperthias and Bulis returned to Sparta. Long after that, however, it rose up again in the war between the Peloponnesians and Athenians, as the Lacedaemonians say. That seems to me to be an indication of something divine.