Atropatene was an ancient kingdom that existed in part of what is now Iran. It existed with Darius III of Persia and Alexander the Great. It started in the 4th century BCE, in what is now northern Iran. Its capital was Ganzak. It ended as a vassal kingdom of the Parthian empire. In 226, the Sasanian empire conquered it. At that time it was probably more of a province of the Parthian empire than a kingdom, even though its rulers were kings.