He ruled as the uncontested king from 208 to 213, but afterwards fell into a dynastic struggle with his brother Artabanus IV,[lower-alpha 1] who by 216 was in control of most of the empire, even being acknowledged as the supreme ruler by the Roman Empire.[2] Artabanus IV soon clashed with the Roman emperor Caracalla, whose forces he managed to contain at Nisibis in 217.[3] Peace was made between the two empires the following year, with the Arsacids keeping most of Mesopotamia.[3] However, Artabanus IV still had to deal with his brother Vologases VI, who continued to mint coins and challenge him.[3] The Sasanian family had meanwhile quickly risen to prominence in their native Pars, and had now under prince Ardashir I begun to conquer the neighboring regions and more far territories, such as Kirman.[2][4] At first, Ardashir I's activities did not alarm Artabanus IV, until later, when the Arsacid king finally chose to confront him.[2]