It can be deduced from Caracalla's request from Vologases for returning the two fugitives that the Romans considered Vologases the actual Parthian power and great shah at the time. About one year later in 216, Caracalla made another excuse to attack Parthia; that time he demanded Artabanus (not Vologases) to give him his daughter for marriage, which Artabanus did not accept and the war started in summer 216. According to that request of Caracalla from Artabanus, it is assumed that Aratabanus gained "the upper hand" in his internal contest with Vologases then, though Vologases' coins were minted until 221–222 in Seleucia. Although the exact path of the Romans' invasion is not known, they certainly conquered Erbil, center of Adiabene; apparently the Parthians avoided a large confrontation; but they applied an offensive policy toward Mesopotamia in early 217. That was the time Caracalla, who was heading to Harran, was killed by head of his security detail Macrinus, who showed his inclination towards peace with the Parthians by "putting the blame of starting the war on Caracalla" and "freeing Parthian prisoners"; but Artabanus demanded the Romans' "relinquishing of the whole Mesopotamia", "rebuilding the destroyed cities and fortresses" and "paying compensations for destroying the royal cemetery of Erbil", knowing of having the upper hand.