By the time extant history hears of him, Aristagoras is already serving as deputy governor of Miletus, a polis on the western coast of Anatolia around 500 BC. He was the son of Molpagoras, previous tyrant of an independent Miletus, and brother-in-law (and nephew) of Histiaeus, whom the Persians had set up as tyrant, but never quite trusted. After general Megabazus presented his complaints about Histiaeus to Darius I of Persia, the latter summoned Histiaeus to his court and detained him at Susa, the main reason being that he wanted a trustworthy advisor. On the recommendation of Histiaeus, the Achaemenids then appointed Aristagoras as the new ruler of Miletus. Aristagoras ruled Miletus while Histiaeus remained in Susa. The assignment was put forward as temporary. Privately, everyone knew that he was being kept under observation away from his troops.