What Was the Grand Strategy of the Roman Empire Before the 3rd Century Collapse?

Entering the Third Century, the Roman Empire, under Hadrian, executed its grand strategy based on the simple concept of perimeter defense. This perimeter defense consisted of legions stationed within fortresses on the Roman frontier, and some were even accompanied by large stone walls (the most famous being that of Hadrian's Wall in North England). Another factor that made it easy for the Romans to adopt this type of perimeter defense strategy was the reputation of it's army, that is, the Roman army was so tactically superior to it's enemies, that it was in itself the biggest deterrent to would-be attackers, not necessarily the fortresses and walls placed along the perimeter.
Another advantage of this strategy was the low cost of army/troop maintenance; in fact, the Roman Empire at one point defended an empire of 50,000,000 people with an army of merely 300,000. This low cost and small army was directly related to the fact that Rome did not employ a central reserve in order to protect the internal empire should the outside perimeter collapse. …