Originally Posted by spqr95
Luttwalk discussed this in his book: "The Grand Strategy of the Roman Empire", though critics disagree whether there was a coherent policy or just an evolution of the army in response to internal and external factors.
I would argue that the military policy of the Empire - certainly after the second century - was entirely reactive and defensive. Mostly the extent of territory and the slowness of communications made it so. There does not appear to be a difference between military and "foreign" policy in the imperial centuries. There was no "foreign office."
Internal politics and civil wars in the third century also drained much energy and many resources from the limits of the Empire in the north. The Northern Peoples were not yet in a position to take full advantage of that, but by 400 it all changed (as did the Northern Peoples).
In the north, as early as the first century, the north - the Rhine and Danube Rivers - became a defensive "line," more rigid before the Dominate, and a defense in depth under the Constantines. The worth of or liability of Britain to the Empire is stiil a subject of debate
In the east, attempts to extend Imperial control to Mesopotamia in the third century were failures, with several defeats and humiliations suffered against Persia. Persia was in the same situation, and neither of them was able to do anything against the other except to slug it out on their peripheries. The eastern territories became a defensive buffer between Persia and the Levant.
The grand strategy was mainly to "hold on to what ya got, and fight who wants to take your stuff." They didn't need a foreign office for that.