In his Dacian conquests, Trajan had already resorted to Syrian auxiliary units, whose veterans, along with Syrian traders, had an important role in the subsequent colonization of Dacia.[17] He had recruited Palmyrene units into his army, including a camel unit,[18] therefore apparently procuring Palmyrene support to his ultimate goal of annexing Charax. It has even been ventured that, when earlier in his campaign Trajan annexed Armenia, he was bound to annex the whole of Mesopotamia lest the Parthians interrupt the flux of trade from the Persian Gulf and/or foment trouble at the Roman frontier on the Danube.[19]