Unlikely frontiers by choice, no decision to halt. Governor who wanted to earn respect by expanding his province frontiers congealed around the orders of the empire: they arose by default Disapproval by writers of unprofitable expansion Britain not really worth it implication not resist profitable expansion, Claudius desires honour of taking Britain. No difficult in asserting little economic r financial reason Aims of war adjusted according to wars balance between defence and conquest. Existing boundary in East never satisfactory not even Nero OK Britain and Germany attempts to expand, efforts to subdue Persians in Mesopotamia more frequent and persisted after warfare in other areas became mainly defensive. Even in these areas not impossible to decide that they might gain the initiative and expand empire rather than stay and repel raids case reign Justinian. Impossible to say quest for defensible borders many wars of expansion, some so called defensive wars can be seen as wars of aggression.
Attempting to gain power and dignity. Again Historians ignore statements at the time fair to attribute Roman decision makers with motives and calculations. Nothing wrong contemporaries of going for glory and honour even Britain and Armenia no practical good. According to Ammianus Julian went to war with Persians for 3 reasons: 1. Punish for previous war 2.
Tired of idleness Certain cements from Emperors explain aims of wars pretty inexplicit. Contemporary sources do not give Emperors the benefit of the door ambitious and vein Modern thoughts that duty emperor to protect his citizens but as we have seen Roman Mesopotamia self-defence yes if assimilate long enough but Tacitus talks of rebels not yet broken by servitude. Protection against all forms of attack not a guaranteed right other priorities integrity of army, sovereignty of empire, safety of certain areas especially in Italy. Protection against foreign aggression less important than sometimes thought.