What was the grand strategy of the Roman Empire before the 3rd century collapse? what Reforms began and how did they change Roman grand strategy, tactics, and military organization?

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 …

…to replace them with the specialized imperial guards of cavalry regiments--although most of these new guards were German, as well. The overall effect of all of Constantine's labor was, unfortunately, the foundation of the Roman army's decline. That is, although under Constantine the army fought with the same spirit and discipline as previous armies, the limited use of the frontier armies led to their gradual decline through both military efficiency and its esprit de corps.