Caucasian-speaking Albanian tribes are believed to be the earliest inhabitants of the region where the modern-day Republic of Azerbaijan is located. Early Iranian settlements included the Scythians (Ishkuza Kingdom) in the ninth century BC. Following the Scythians, the Medes came to dominate the area to the south of the Aras River. The ancient Iranian people of the Medes forged a vast empire between 900 and 700 BC, which the Achaemenids integrated into their own empire around 550 BC. During this period, Zoroastrianism spread in the Caucasus and in Atropatene.