I never finished The Decline and Fall of the Roman Empire, but I got about half way through that prodigious tome. I did manage to finish The Ottoman Empire, which itself was quite a beast.
What you saw in both cases were nations that practiced offensive defense. That is, if they felt a country on their borders was a threat or would become one in the near future, they would attack and conquer it. If they thought they needed some buffer space on their borders they would also attack and conquer neighbors so any invader of them would have to go through less-valuable conquered territory first.
As near as I can tell, that was a pretty common practice up until the age of nuclear weapons. In fact, the Warsaw Pact was a buffer zone for the Soviet Union. The concept is part of a bygone era, one in which the United States never really engaged. I suppose you could consider the Mexican-American War to have been that sort of thing, but not being up on that conflict, I couldn't say.
It seems to me that we're re-entering that world. A nuclear Iran and a bellicose, nuclear North Korea would long ago have triggered the Ottomans and Romans to take self-preserving action. We didn't and now we're sitting around trying to figure out how we can make it all go away. That doesn't seem to have much hope for success.
Human nature hasn't really changed in 2000 years. Maybe the Romans and Ottomans were on to something.