Herodotus: The Father of History

Herodotus, the Peloponnesian War, and Thucydides (1x23)

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For whom, and amid what circumstances, is Herodotus writing? Does he take sides in the conflict between Athens and Sparta? How does knowing his work shed light on the very different project that his younger contemporary Thucydides undertakes in writing on the Peloponnesian War?