who was the common enemy of sparta and athens

Question

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Answer 1

When King Xerxes of Persia invaded Greece, Athens and Sparta teamed up to defeat the Persian invasion.  However, the Greek city-state of Thebes fought on the side of Persia during that war.  Persia certainly was the main common enemy of Athens and Sparta during that war.  If you must provide exactly one best answer, then it would be the Persian Empire.