We are hard pressed to understand the events of Caracalla's Parthian war, including the role Armenia played in the conflict, because of gross inadequacies in our sources. A careful analysis suggests that Caracalla intended to annex Armenia but never saw the project through. His intentions can be gauged by his treatment of Edessa, for whose annexation the evidence is more solid. Caracalla was trying to secure his rear, from Osrhoene to Armenia, in preparation for a full-scale Parthian war. Because the goal of stabilizing Armenia proved elusive, given local hostilities, Caracalla had to scale back his plans.
Patterson, Lee E., "Caracalla's Armenia" (2013). Faculty Research & Creative Activity. 59.