I'd like to recommend a book that I finished a short while ago: The Grand Strategy of the Roman Empire by Edward N. Luttwak, a noted writer on modern strategy. The book examines changes in the defenses of the Empire from the first to the third centuries and tries for account for them by positing three grand strategies adopted consecutively by the Empire.
Luttwak's arguments are persuasive, although he irritates the reader by continually denigrating other authors as mere "Clausewitzians". For me, his greatest contribution is examining, in depth, the "big picture" that faced the Romans. Most of us Romanophiles, I suspect, fall in love with the details of the Roman army: its camps, equipment, arms, marches, walls, tactics, battle cries and commands, etc. But until I read Luttwak's book, I had only seen very short discussions of Roman grand strategy, just enough to be thought-provoking but hardly satisfying.
You might not agree with everything Luttwak says (I didn't), but you won't regret reading the book.
Tergestus